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As it happened: Test day four
By Pablo Elizalde, Matt Beer, Edd Straw, Glenn Freeman, Ben Anderson and Dan Cross
The live commentary has ended. No further updates will be posted.
05:38 Good morning and welcome back to AUTOSPORT Live for the fourth and final day of the second 2014 winter test in Bahrain.

05:42 As the initial teething troubles pass, the patterns are getting clearer - and yesterday was another day when Mercedes and McLaren(-Mercedes) had plenty to be pleased about, while frowns grew deeper at Red Bull and Lotus.

Bahrain test day three report - Hamilton fastest

05:44 Here's the driver line-up we're currently expecting for today. There's already been one tweak with Caterham giving Marcus Ericsson the morning and delaying Kamui Kobayashi's return to the cockpit until the afternoon.

Red Bull – Daniel Ricciardo
Mercedes – Nico Rosberg
Ferrari – Kimi Raikkonen
Lotus – Pastor Maldonado
McLaren – Jenson Button
Force India – Sergio Perez
Sauber – Adrian Sutil
Toro Rosso – Jean-Eric Vergne
Williams – Felipe Massa/Felipe Nasr
Marussia – Jules Bianchi
Caterham – Marcus Ericsson (am)/Kamui Kobayashi (pm)

Felipe Nasr05:47 Some news from overnight (announced at 3.30am Bahrain time, but - more to the point - primetime 9.30pm Brazilian time) to share with you: Felipe Nasr has been confirmed as the Williams test driver for 2014.

Nasr gets Williams test and reserve seat

06:01 The final day of this test has started. Marcus Ericsson, Nico Rosberg and Jenson Button are the first drivers to take to the track.

06:04 While the Renault-powered Caterham stops at the end of that installation lap, Mercedes-engined Rosberg and Button have stayed out. A sign of confidence in the machinery from the Mercedes and McLaren teams?

Jean-Eric Vergne pits after one lap in the Toro Rosso.

06:05 Marussia, which only completed four laps yesterday as a result of having to perform an engine change, states that it is "hoping" to start its programme with "some good short runs".

06:10 Kimi Raikkonen takes the Ferrari out. The Finn and his team are playing catch-up today after losing several hours of running with a telemetry problem yesterday.

Speaking to the media after yesterday's running, Ferrari engineering director Pat Fry said that it wasn't the telemetry system itself that caused all the problems, but the complexity of everything else on the car that was involved in working to fix it.

"It was just a simple silly thing we got wrong," he said. "There's nothing new, it's the same thing as last year. We just need to get things sorted a bit better.

"The cars are so complicated, it takes a while to get in and find the problems. It's harder getting to the bits to change them than changing the bits themselves."

06:15 No cars on track at the moment. So far we've seen Button, Rosberg, Ericsson, Vergne, Perez and Raikkonen.

06:16 New Williams signing Felipe Nasr is suited up and ready to go out in the FW36.

06:17 Ericsson breaks the silence on track, and he is followed out of the pits by Daniel Ricciardo in the Red Bull.

06:20 Ericsson, who has been added to the Caterham line-up today to share running with team-mate Kobayashi, is the first to set a time.

06:21 First red flag of the day. The Caterham of Ericsson has stopped.

06:21 Some kind of car problem. He's getting out of the car now and a track vehicle is already in attendance.

06:23 The JCB is now with Ericsson's Caterham ready to recover the car when given the go-ahead to do so by the team.

06:27 The Caterham, which stopped on the backstraight, is up on the flatbed truck and now being covered by members of the team.

06:30 As the truck makes its way through the back of the paddock, the track is now clear. However, based on what the clerk of the course Fayez Ramzy told AUTOSPORT Live yesterday about the recovery process, we're only likely to get a green flag once the Caterham is back in its garage.

06:34 The sight of an empty flatbed truck disappearing from the pitlane is followed by the track re-opening.

Rosberg heads straight back out in the Mercedes.

06:39 Vergne's first timed lap sends him to the top of the standings.

Pastor Maldonado, Lotus, truck06:40 In case you missed the news last night, don't expect to see anything of the troubled Lotus team just yet, as it is waiting for parts to arrive from Enstone, having become the first squad to very publically suffer due to Sakhir being a lot further from the UK than Jerez.

Lotus set for late start on final day

06:44 Perez will be in and out of the pits a lot this morning, according to his Force India team.

@clubforce: "First task on the job list in Bahrain is some aero work. @SChecoPerez out on track now. Lots of very short runs planned."

06:45 And with that, the Force India breaks the silence on track.

06:54 Williams new-boy Nasr heads out for his first lap, joining Perez on track.

06:56 Nasr gets his first time on the board for Williams, putting him second fastest behind Vergne.

06:58 Another improvement for Nasr moves him to the top of the pile. It's not much of a pile at the moment, as only Vergne and Ericsson have also set times.

07:00 Rosberg joins that list of drivers to set a time, slotting into second with his first effort.

07:01 With the Mercedes heading for a potential fastest time on its next lap, the Williams of Nasr lowers the benchmark at the top of the order.

07:02 The Brazilian's effort is still no match for Rosberg, who goes three seconds clear of the Williams.

Button is next through, just over a second adrift of the fastest time.

07:03 Button improves again - including an overall best through the middle sector - but it's not enough to knock Rosberg out of P1.

07:05 Rosberg, Button and Nasr are the drivers keeping the top of the timing screens busy at the moment, but we are still getting occasional short appearances from Perez (as promised by Force India) and Kimi Raikkonen has just set his first time of the day.

07:06 After being 0.2s down on his best through the first sector, Rosberg sets a new personal best in sector 2 and then pits the Mercedes.

07:07 So far we've seen eight cars today - the six on our leaderboard plus Perez's Force India (5 laps) and Ricciardo's Red Bull (1 lap).

No sign of Lotus, Sauber or Marussia yet with just over an hour of today's running already in the books.

07:10 Button manages a personal best first sector on his latest lap, but in the end he comes up 0.010s shy of his current quickest effort.

07:14 Nasr's outing for Williams is about more than just giving the new test and reserve driver some F1 mileage. The team tells us that it is carrying out aero work this morning, the the car has been sufficiently caked in flo-vis (where required) as part of that programme.

Marshals07:17 Remember the half-hour delay before the pitlane was opened on the first morning of the test, which was down to insufficient marshals being on post?

EDD STRAW asked clerk of the course Fayez Ramzy about the reasons behind the delay. He explained that a number of flag marshals were late on post because of a crash on a road close to the track.

"We had a problem with the deployment of the marshals at the start of the day, which was because of a road accident outside the circuit in the morning," he said.

"There were no injuries and it was right on the road as you approach the track, there was a five-car accident. It was a simple road traffic accident. Some were marshals and because they were delayed we could not start.

"Nothing starts until we have full deployment and there are enough marshals. It was the flag marshals only, as the firemen, the ambulances, the fire trucks, the medical team were all in place.

"By 0930 the marshals were all in place as well but before we could start we had to send the course car round to check. You cannot take risks.

"Then we informed the teams that we would extend running until 1730 to make up for it."

07:20 It's been a few minutes since we've had anything on track, and Perez heads back out in the Force India for what we would assume is another of the team's planned short runs.

07:21 Caterham mechanics have descended on the CT05 to work out what has halted Ericsson's running after just four laps.

@CaterhamF1: "The car's back, the bodywork and floor are off and investigations continue into the cause of the issue. More information when we have it....

"Testing is testing, that's why it's called testing...."

07:22 Yes, that was a very short run for the Mexican. Perez returns immediately. Rosberg heads out, setting a new overall best time through the second sector on his outlap.

07:25 More overall bests from Rosberg, this time through sectors 2 and 3 as he lowers today's benchmark.

07:28 Much like the early-morning programme we saw from Rosberg's team-mate Lewis Hamilton yesterday, the German follows up his new best lap with a much slower lap, then goes for it again.

However, his second 'quick' lap of this run is 0.7s down on his fastest time.

07:31 We get our first sighting of the Lotus E22 for today. The team was concerned about how late it might be forced to start its programme after a week of problems, but it has still beaten two other teams to the punch this morning.

You can catch up on Lotus's overnight concerns here:

Lotus set for late start on final day

07:33 Pastor Maldonado stays out to pass the pits at the end of his out-lap.

Fair to say he was taking it pretty easy as he cruised down the start/finish straight.

07:34 Maldonado pits at the end of that second lap, so there's no time on the board from Lotus just yet.

Romain Grosjean07:35 Lotus remains the top team most in need of an encouraging breakthrough.

Its late start to 2014 and a troubled first few days of running left Romain Grosjean pretty downcast on Thursday night when he'd finished his sessions. GLENN FREEMAN's analysis gives a flavour of the mood:

Is Lotus running out of time?

07:37 Here goes Rosberg again. Let's see if this is another push-cruise-push run.

Maybe not - we have a red flag and the Mercedes is the only car on track.

07:38 It looks like the F1 W05 has stopped at Turn 2. The recovery vehicles have reached Rosberg already.

07:38 Maldonado's first public day driving for Lotus yesterday featured its fair share of problems, mainly related to the gearbox.

The team was forced to delay its running this morning as a result, in order to fly parts over to Bahrain from Enstone.

@Lotus_F1Team: "10:30 am local time, @Pastormaldo takes to track - parts from Enstone having arrived and fitted overnight and finished this morning."

07:40 Perez has finished his early-morning aero work for Force India. Time to work on setting up the VJM07.

@clubforce: "10 laps and our aero work is complete. Checo steps out of car as we get it ready for today's baseline set up programme."

07:44 The Mercedes is being transported back to the pits. At least it doesn't have far to travel.

07:47 Rosberg stopped twice on track on his previous day in the car earlier this week. The Mercedes package has largely been reliable, and he told AUTOSPORT that the signs of a few problems in Bahrain are not a cause for concern.

"It's the result of doing many laps," he said. "When you start to do that many laps, naturally you'll start to see the problems in the car."

07:47 The F1 W05 is now being returned to the Mercedes garage, so we should be back to the action shortly.

07:50 Back to green here. Ferrari sends Kimi Raikkonen out immediately.

07:56 Improvements from Button and Nasr, who are still second and fourth fastest respectively. Button's now in the 36s, Nasr almost in the 38s.

08:01 Rosberg returns to the action very shortly after causing that earlier red flag. Mercedes explains that there was nothing wrong with the car: "just a stall following a practice start in the pitlane!"

Slightly strange one that, as Rosberg seemed to be pretty far from the pit exit by the time he came to a halt.

08:01 Tiniest of improvements from Nasr, shaving just a couple of hundredths of a second off of his best so far this morning.

08:03 Rosberg teases with a new outright best through sector 1, but in the end he backs off and crosses the line nearly 14s adrift of his current fastest lap.

08:05 This time Rosberg stays on it for the whole lap, and the result is our first 1m33s effort of testing here in Bahrain.

08:07 Ferrari's Pat Fry admitted yesterday that the team has "got a bit of catching up to do" after losing a few hours of running yesterday to telemetry problems on the F14 T. Given that we've already had a couple of red flags today that have eaten into some of the track time, the team's Twitter account has taken to a bit of number crunching:

@InsideFerrari: "Two red flags so far today. Each time you lose about 20 minutes of time. 12 days of testing per team. 6 per driver. Will it be enough?"

08:08 For those just joining us, here's a quick summary of the day so far:

*Rosberg becomes first driver at this test to lap under 1m34s
*New Williams reserve Felipe Nasr drives the FW36
*Lotus starts around 90 minutes late
*No sign of Sauber or Marussia yet, only one lap from Red Bull

Sergio Perez08:10 Perez sets his first timed lap of the day. EDD STRAW is with the Force India team at the moment and has learned a little more of its programme for today:

"Aero mapping this morning, and set-up baseline changes have just been carried out for Perez.

"The team is planning a series of long runs this afternoon."

08:18 We've had a few minutes of silence here, which has been brought to an end by Button starting a run in the McLaren.

08:20 Got a question for AUTOSPORT's team in Bahrain? Email or Tweet using #autosport and we will go through some of them later today

08:21 Button sets a 1m38s, and is joined on track by Rosberg's Mercedes.

08:24 Another new benchmark for the week from Rosberg. He's down to a 1m33.2s now - will we see a 1m32s before the end of the day?

08:24 That was one flying lap and back to the pits for the Mercedes.

08:30 Raikkonen finds six tenths (and looks to be improving again). Currently he's still third.

08:31 No improvement in the end on that next lap for the Ferrari, as he crosses the line one second shy of his best.

08:36 We have a red flag. Perez, who was on course to set a new best time for the day, is the only car not to make it back to the pits.

08:38 So far today it has been the Mercedes powered teams and Kimi Raikkonen's Ferrari doing most of the leg work out on track.

While we have seen Toro Rosso, Caterham, Lotus and Red Bull out on track, their appearances have been very brief.

Still no sign of Marussia or Sauber either.

08:41 While we wait for the Force India to be returned to the pits, here's the lap count for today:

Button (McLaren) 29
Nasr (Williams) 23
Rosberg (Mercedes) 22
Raikkonen (Ferrari) 20
Perez (Force India) 19
Vergne (Toro Rosso) 6
Ericsson (Caterham) 4
Maldonado (Lotus) 2
Ricciardo (Red Bull) 1
Bianchi (Marussia) 0
Sutil (Sauber) 0

08:42 If you want that lap count by engine manufacturer, it looks like this:

Mercedes 93
Ferrari 20
Renault 13

08:47 We have a recovery vehicle in the pits to deliver a VJM07 back to Force India.

08:49 Our F1 Editor EDD STRAW is taking advantage of this break in the action to make his way to the pits. We'll pass on any snippets of info he discovers while he's there.

08:50 The track is open again, and it's the Red Bull of Ricciardo that is first out onto the track.

08:52 Raikkonen, Nasr and Button follow the Red Bull out of the pits, Ricciardo stays on track at the end of his outlap.

08:52 The Lotus of Maldonado heads out too - another car we have hardly seen today.

08:53 Ricciardo's first time of the day gets him into the 1m41s.

08:55 @clubforce: "Sergio's car is back in the garage, where we will investigate the issue that led to it stopping on track."

08:55 Ricciardo goes faster again (a 1m40s this time), while Button finds nearly two seconds to get himself a bit closer to Rosberg's pace-setting time.

08:57 While Ricciardo keeps going, it's worth noting that Maldonado's appearance on track was a fleeting one and he is back in the pits.

09:00 Here's an observation on Mercedes - and that 1m33.283s lap from Nico Rosberg in particular - from ex-F1 designer (and AUTOSPORT technical expert) GARY ANDERSON:

"I don't think there was much fuel in the Mercedes when he was doing that qualifying simulation, but the team has dissected the new regulations for 2014 and made major progress over the winter to achieve these lap times. That's a credit to all in Brixworth and Brackley."

09:01 Ricciardo improves to a 1m39.8s. We haven't seen many Renault-powered laps faster than that all week, but it's still a long way adrift of what the Mercedes teams in particular are capable of.

09:06 EDD STRAW has returned from the pits, reporting that when the Lotus made that brief appearance just now the car had a lot of black tape on the right-hand sidepod where two pieces of bodywork join.

He couldn't see the left, so we can't be sure if it was on both sides.

09:08 Ricciardo is still going, taking the RB10 into double figures for today but not troubling his personal best time from earlier in this run.

Raikkonen and Nasr are also on track at the moment.

09:10 @Marussia_F1Team: "We've spent the morning so far resolving a systems problem and now we're aiming to be back on track very soon"

09:12 All quiet on track again, as we have 11 cars in the pits.

09:15 While there's little going on, here's a recap of the day so far:

*Rosberg sets fastest time of the week for Mercedes
*New Williams reserve Felipe Nasr driving FW36
*Sauber and Marussia yet to run
*Only a handful of laps for Lotus and Red Bull

To follow-up on that final point, we've hardly seen the Lotus at all today (4 laps, no time), but Red Bull has just completed its first proper run of the day and now has 13 laps on the board.

09:20 Williams has too many 'Felipes' on its books with the arrival of new test driver Nasr:

@WilliamsF1Team: "Even #Massa is getting confused. He keeps hearing 'Felipe' on the radio and panics that he is supposed to be doing something! #Bahrain"

09:21 Bianchi takes the Marussia out for its first lap of the day. The team reports that this is only an installation lap, and then "short runs" are in the plan.

09:30 Sauber has given an update on its problems to EDD STRAW. Here's what the team said:

"There was a problem with an interior part of the monocoque. Therefore, we changed the chassis. The crew is pushing to get the car back on track this afternoon."

That news has prompted a couple of raised eyebrows from Straw, who notes that's a very big job.

09:31 Maldonado sets his first time in the Lotus. Eagle-eyed EDD STRAW also spotted him kissing the grass on the outside of the main straight as he crossed the line. How strange!

09:35 @Lotus_F1Team: "Current laps to run through a variety of settings and programmes. Sounds like lots of button pressing and knob twisting for @Pastormaldo"

09:36 The Lotus of Maldonado is currently sharing the track with Nasr's Williams.

09:37 Talking of Nasr, it's worth confirming that he will be in the Williams all day today. The team revised the programme for its race drivers yesterday, splitting the running between Valtteri Bottas and Felipe Massa.

09:45 Marcus Ericsson's half-day in the Caterham is over, and car problems limited him to just four laps:

@Ericsson_Marcus: "Happy to get the chance to drive again this AM. But unfortunately we had a problem after 4 laps which meant end of the day for me. Now the guys @caterhamf1 are pushing hard to get the car ready for the PM session when @kamuikobayashi is in the car. Still I have now done over 100 laps and I have learned so much! Can't wait to drive again next week!"

09:45 Nasr is still circulating, and Bianchi heads out for a second time today.

09:55 Raikkonen is back on track and has improved his best time, although it's not yet close to troubling Button's P2 effort.

10:00 Maldonado is out too for Lotus, currently lapping in the 1m45s (just).

10:02 Everyone is in the pits now, so the track is silent with just under four hours remaining.

10:11 While we've got a lull in the on-track action, here is the first of our reader questions, answered by F1 editor EDD STRAW.

Graeme Dahl asks: In testing marshals have been told not to handle cars, and have to wait for teams to arrive. What will happen at a GP when a car stops?

EDD STRAW: "That’s correct, the teams have instructed the circuit to wait until they have taken mechanics to the car before they touch it. This is a very wise safety precaution given the new engines and their potent electrical systems.

"Right now, it’s an ultra-conservative approach and a logical one. The track is open from 0900-1700 so there is no reason to take any risks with the safety of marshals where live electricity is concerned.

"Once the season starts, things will be slicker. The main reason for this is that teams will have plenty of experience running the cars and will have proved all the requisite safety systems, including the accuracy of the car safety signalling system.

"Doubtless marshals will be cautious once the season starts, but the recoveries will doubtless be done as efficiently and rapidly as ever."

10:11 Here's our round-up of the morning's events so far:

red button Rosberg keeps Mercedes on top on final morning

10:12 Nasr back out on-track in the Williams. He's now clocked 50 laps on his first outing in an F1 car.

10:15 Rosberg back out in the Mercedes now and straight into the 39.0s - a lap time the Renault-engined teams can but dream of at the moment.

10:19 Raikkonen back into the fray for Ferrari now too.

10:21 As Mercedes continues to lead the way, Lewis Hamilton admits the team is ahead of what it was expecting:

Hamilton says Mercedes F1 team 'way ahead' of what he expected

10:23 Raikkonen just a tenth shy of his best on that first flying lap, before dropping back to a 38.4.

10:27 Raikkonen in the 39s on that last lap as he overtakes Rosberg's Mercedes on the start/finish straight.

10:30 Things are fairly quiet on-track at the moment, so let's have another reader question.

Kathy Walker asks: Do you know if any teams have enough race distance data to predict if the 100kg allowance will actually last?

EDD STRAW: "The teams have built up plenty of information about fuel economy and done a lot of work on the different strategies that can be adopted to make the 100kg allowance last.

"In the case of Mercedes, they did a race simulation run at Jerez that was actually five laps longer than a grand prix distance, and those who have been able to put together ‘normal’ programmes will have a decent amount of data.

"The question is whether they have enough data to work out how to complete the race distance in the quickest possible way, and manage every possible race situation?

"The answer to that is a resounding, 'not yet!'"

10:31 Raikkonen brings the Ferrari back into the pits after that short run, but Ricciardo has headed back out after a 1h20m delay in the Red Bull.

10:34 The Red Bull is straight back to the pits on that out-lap, while Raikkonen has been back out and now stopped properly in the pits.

Just Rosberg circulating in the Mercedes right now, doing 1m42s.

10:39 Not content with bringing you the latest developments, controversies, opinions and laptimes from Bahrain in text format, the AUTOSPORT team also participated in The Racers' Edge with F1 Racing last night.

Here's a round-up from day three:

10:43 Maldonado's Lotus joins Rosberg on-track, after 40 minutes spent in the pits.

10:46 Maldonado sets personal best times in the first and second sectors of the lap, but crosses the line nearly two seconds shy of an overall improvement.

Maldonado10:52 Maldonado still circulating in the 44s, Rosberg in the 40s. They have the track all to themselves at the moment.

10:56 It's taken him a fair few laps to get there, but Maldonado jumps to seventh with a three-tenth improvement.

11:00 He's much slower on his next two efforts though, dropping back into the 42s and then the 44s.

11:01 Button's McLaren joins Rosberg on-track as Maldonado dives for the pits in the Lotus.

11:03 BEN ANDERSON was hoping we'd see our first 1m33s lap of the test today, and he was right. Here's what he wrote yesterday about how quick the 2014 cars will get:

F1 2014 is not too slow

11:03 Let's have another reader question now, courtesy of Garry Webster, who asks: Just how hard are Mercedes pushing? They seem to be in a deferent league to most others though McLaren also looks strong.

EDD STRAW: "It depends on your definition of pushing. Rosberg’s fastest lap of today was mightily impressive and there is no doubt that the car is quick. But I’m sure there’s even more to come and we haven’t seen the best of a number of other quick cars by way of comparison.

"As you say, the McLaren is in a similar ballpark and we’ve seen decent times from all four Mercedes powered cars.

"In terms of how advanced it is with its programme, things are looking excellent for Mercedes."

11:04 Button pumps in a 1m35.0s lap on his first flying effort, just under a tenth shy of his best effort from this morning.

11:06 EDD STRAW reports from the pitlane that Button is on a soft-tyre run in the McLaren right now, which would explain his quick first flying lap.

11:08 A 1m36.0s lap for Button last time around. The 2009 world champion is definitely trying, locking up at the slow left-hander onto the back straight in front of EDD STRAW.

11:10 Button heads for the pits now as Rosberg recovers from a brief off at the left-hander coming on to the back straight.

It appears he's struggling on the brakes now in the Mercedes, according to EDD STRAW.

11:13 Not much for EDD STRAW to watch trackside at the moment, as everyone is in the pits bar Rosberg.

11:16 Formula 1 newboy Nasr decides to break the Mercedes stranglehold on track time by heading back out in the Williams. He's been in the pits for nearly two hours...

11:20 ... But he's straight back in again.

11:26 Button and Maldonado both back out now. Button is straight into the 41s for McLaren.

11:27 The session has come to a halt. Rosberg and Button have returned to the pits, so it's Maldonado's Lotus that caused the stoppage.

11:30 EDD STRAW reports a marshal is replacing a bollard on the inside of the exit of the left-hander coming onto the back straight. This is likely the reason for the red flag, as Maldonado's Lotus is already back in the pits and there is no sign of the mobile crane...

11:35 Let's take this opportunity to answer another reader question.

Sergio Leal, a Brazilian living in the Netherlands, asks: "I'm very happy to see a young promising Brazilian driver having an opportunity in F1. But, what does Nasr's signing mean for Susie Wolff? Is she out of Williams?"

EDD STRAW: "AUTOSPORT understands that Susie Wolff remains in the Williams fold although her precise role has yet to be finalised.

"Certainly, there is no expectation that she is out of the team in any way."

11:38 We have exceeded the 200-picture mark in our high-res test gallery this morning. Give your eyes a treat by clicking on the following hyperlink:

null Bahrain F1 test in pictures

11:38 No one is able to add to their tally right now, so this is a good opportunity for a lap-count with just under two and a half hours remaining here in Bahrain.

Rosberg (Mercedes) 72 laps
Nasr (Williams) 56 laps
Raikkonen (Ferrari) 53 laps
Button (McLaren) 47 laps
Maldonado (Lotus) 32 laps
Perez (Force India) 19 laps
Ricciardo (Red Bull) 15 laps
Vergne (Toro Rosso) 6 laps
Ericsson (Caterham) 4 laps
Bianchi (Marussia) 2 laps

11:40 Go again in Bahrain. Marussia is reporting that Bianchi is ready to head out again after an engine change.

@Marussia_F1Team: "Jules is in the car, ready to head out of the garage."

11:41 But Button and Rosberg are the first drivers to head back out of the pits.

11:47 For those of you just joining us, here are the main headlines so far from day four in Bahrain:

*Rosberg becomes the first man to lap in the 1m33s for Mercedes
*Nasr makes his F1 driving debut after signing for Williams
*Raikkonen does over 50 laps for Ferrari after yesterday's setback
*Sauber chassis problem prevents Sutil from running
*Technical problems for Caterham curtail Ericsson's day too

11:50 Button and Rosberg are the only two drivers on-track at the moment. The Mercedes is lapping in the late 40s, the McLaren in the late 41s.

11:51 Bianchi's Marussia came straight back into the pits on that out lap.

11:53 Rosberg has continued his race-run for Mercedes following that red flag. He's now clocked 50 laps with one (scheduled) pitstop and is lapping consistently in the 1m40s.

11:58 Nasr's back out in the Williams, and Sauber has finally managed to send Adrian Sutil out for his first lap. Installation done, he's returned to the pits with two hours to go here.

Nasr11:59 EDD STRAW reports a big lock-up for Nasr at Turn 1.

"Several big rear-end wobbles through there and Turn 2."

12:06 A seven-minute delay while the team checks the car, then Adrian Sutil is back out for his first proper laps here in Bahrain.

12:08 Button, Nasr and Raikkonen are also circulating, as Rosberg's Mercedes pits. Bianchi's Marussia has not been seen again since that post-engine change installation lap...

12:10 Sauber is relieved to have got its car ready to run before losing out on a whole day of testing.

@OfficialSF1Team: "C33 threw a bit of a tantrum this morning and was incredibly shy. Glad our crew convinced it to get back on track."

12:10 Sutil is back into the pits now too. Only three laps on the board and no time set yet. Button and Nasr come in as well, leaving the track all to Raikkonen.

12:14 Raikkonen's Ferrari is in too now, so the track is silent with 95 minutes of testing remaining.

12:21 Time for another reader question now, this time from Michal Hryszko, who asks:

"Don't you think this may be the year to get rid of DRS?

"In 2014 we can expect less downforce, worse power/grip ratio at the corner exit, plus longer breaking zones, giving even more chance to make a move. A move that could again be something, not just a worthless change of position.

"Sure it is to early to make a definite conclusion, but I think it is worth discussing."

EDD STRAW: "DRS remains in the regulations for this season with its use permitted only in the allocated zones for both qualifying and the race.

"For this year, because of the anticipated reduction in overall downforce, the gap in the rear wing increases from 50mm to 65mm.

"The idea behind the DRS was to counteract the fact that there is a significant downforce loss for a car following in another’s wake which can make it impossible to be close enough to mount a challenge.

"DRS passes are often dependent on hanging on close enough to the car in front despite a big disadvantage in the previous corner, and are often not as worthless as some argue.

"That said, at times the length of the DRS zones have been too generous and passes have become straightforward.

"While the braking zones will be longer, they will still be pretty short in absolute terms, reducing the available time to make a pass.

"Exit speeds could be compromised more by the difficultly in putting the power down. But that will be down as much to engine characteristics as driver skill."

12:23 Raikkonen is the first man to break the recent silence as he heads back out onto the track in the Ferrari F14 T.

12:28 Raikkonen is circulating in the 1m40s as Maldonado ventures back out in the Lotus.

12:37 Force India is done for the day after just 19 laps for Perez. The official line is "drivetrain issues" but AUTOSPORT understands a gearbox problem has curtailed running for the VJM07.

12:41 Nasr breaks into the 37s for Williams on his 68th lap. That effort puts the Brazilian within 0.8s of Raikkonen's Ferrari.

12:46 Rosberg appears to have finished his race run for Mercedes before the close of the first Bahrain test.

Many rivals are tipping the three-pointed star as championship favourites this season, but Rosberg's team-mate Lewis Hamilton wasn't getting carried away after his running yesterday.

"It's good to have positive feedback; the people I'm working with deserve the compliments.

"But it's far far too early. I have no idea where we stand compared to others.

"People can bring upgrades and fix issues and we're still finding things to work on every day. It's still very much a work in progress."

12:47 Maldonado gets down into the 1m40s for Lotus with not much more than an hour left to run.

12:51 Caterham is still hopeful of getting Kobayashi out before the close of play.

@CaterhamF1: "Engine fired up, floor going on. Will we be back out before the end of day 4? Bloomin' well hope so after the effort put in today."

12:52 Another stoppage. Rosberg is the only driver who hasn't made it back yet...

13:03 The Mercedes is back in the pits and the Bahrain crane is making its way back to the paddock, so we should be back underway very soon.

13:03 Green again. But there's no rush for the pitlane exit with less than an hour to run.

Kobayashi13:04 Kobayashi is the first to venture out in the repaired Caterham.

13:10 The Japanese driver gets the CT5 on the board with a 1m47.851s and lowers his own benchmark by 3.6s on the next lap. Maldonado has also improved in the Lotus, now down to a 1m38 and up to fifth.

13:12 The Lotus E22 has endured a troubled start to life here in Bahrain, but that effort from Maldonado stands as the quickest lap by a Renault-powered car so far.

13:14 Mercedes is looking into that Rosberg stoppage that caused the latest red flag. The team also confirms that it will not run again today, so that's this four-day test over for the Silver Arrows.

13:15 Given that Rosberg has 89 laps to his name and sits comfortably at the top of the timesheets, Mercedes will probably still be pretty happy with its day's work.

13:15 Marussia is hopeful of getting Bianchi out before the end of the day.
EDD STRAW has an update from the team:

"Marussia is going back out they hope. Fuel system problems today, but some bits were flown out and arrived earlier today, so they are going to try and get the car back on track at least briefly."

13:16 Vergne has done hardly any running in the Toro Rosso today, but his ninth lap gets him through the 1m41s barrier and into the 40s.

Bahrain13:21 One of the great things for the teams about testing in Bahrain is that - aside from a bit of wind from time to time - the weather isn't really a topic for discussion here.

However, with some light cloud cover coming over the circuit this afternoon, track temperature has fallen from around 40°C to 25°C in the last 80 minutes.

13:26 It's been a long time since we saw Daniel Ricciardo and the Red Bull RB10. Speculation is rife in the paddock that the team is done for the day, but EDD STRAW is currently in the pits trying to find some concrete information.

He's not having much joy down at the team's garage right now though: "Doors closed and no external activity.

"There is the sound of plenty of work going on in the pit though. Apparently the team is trying to get the car back out before the end of the day."

13:28 We've already had it confirmed that Mercedes will not be running again today. The car is now up on the stands in the garage while the team investigates the issue that forced Rosberg to stop.

EDD STRAW adds that the F1 W05 has lots of aero paint (or flo-vis) on the suspension and barge boards, and the team are looking under the car at the rear.

13:31 Only Kobayashi's Caterham is currently circulating as we enter the final half an hour of this week's testing.

13:34 Kobayahi is into the 43s now, having just posted his best lap of the day.

13:38 A surge of activity in the last few minutes has been halted by another red flag, this time it's Adrian Sutil's recalcitrant Sauber that's to blame.

13:47 The cars are all back in the pits now so we should be green again soon. Less than 15 minutes to go now.

13:52 Back underway in Bahrain. Raikkonen, Nasr, Maldonado, Vergne, Kobayashi and Bianchi have all headed out with eight minutes to go.

13:55 Raikkonen's Ferrari has stopped on the exit of Turn 4 so the session has been red flagged with five minutes left to run.

Kimi Raikkonen13:56 The Ferrari has come to a halt at a strange angle. It's not clear whether he's broken down, spun or even hit something.

13:57 @Marussia_F1Team: "Today we have experienced a continuation of some component reliability issues. So, we opted for a change of strategy. Instead of persevering, we've been working feverishly to fit some revised parts which had arrived for next week and shake them down today."

14:00 Recovery vehicles have arrived to pick the Ferrari up and take it back to the pits. Running out of time to resume the session now.

14:00 No time for a restart, so that's it from the fourth day of pre-season testing in Bahrain.

14:18 Thank you for joining us on AUTOSPORT Live this week.

The next Live action on the site will be Race Centre Live coverage of the NASCAR Sprint Cup season opener at Daytona, which begins at 17:00GMT tomorrow - February 23.

AUTOSPORT F1 Live will be back for the third and final week of 2014 testing in Bahrain on February 27.

And keep an eye on the site over the weekend for more analysis features from this test.

P Driver Team Time
1  RosbergMercedes 1m33.283s
2  ButtonMcLaren 1m34.957s  +1.674
3  RaikkonenFerrari 1m36.718s  +3.435
4  NasrWilliams 1m37.569s  +4.286
5  MaldonadoLotus 1m38.707s  +5.424
6  PerezForce India 1m39.258s  +5.975
7  RicciardoRed Bull 1m39.837s  +6.554
8  VergneToro Rosso 1m40.472s  +7.189
9  KobayashiCaterham 1m43.027s  +9.744
10  EricssonCaterham 1m45.094s  +11.811
All timing unofficial. Updated: 13:34 GMT
Clear skies High Temp: 23°C / 73°F
Track: Dry
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