Ferrari set the fastest lap for the second time in four days as the first pre-season test came to an end.
Kimi Raikkonen was 0.897 seconds quicker than Red Bull's Max Verstappen at the Circuit de Barcelona-Catalunya.
Lewis Hamilton did not run after Mercedes discovered an electrical problem with the car, while team-mate Valtteri Bottas was eighth-fastest.
Williams were unable to test because of damage done to their car in a crash by rookie Lance Stroll on Wednesday.
A chassis needed repairing after the 18-year-old Canadian spun into the barriers in his third off-track moment in two days.
Williams said they "aimed to be back on track" for the start of the second and final test on Tuesday.
Mercedes were afflicted by reliability problems for the first time in what has been an otherwise impressive first test, with the team stopping Bottas early when it discovered an anomaly in the computer data.
Englishman Jolyon Palmer continued what has been an encouraging first test for Renault with the third fastest time.
"I am driving with such a big smile on my face because the car feels so nice to drive," Palmer said. "Everything at the moment is pointing in a good direction."
Wet tyre testing
The final day of the first test was intended for teams to try out a new range of wet tyres designed by Pirelli after criticisms last year.
Drivers had asked for changes to the 'extreme' tyre, used in the heaviest rain, because it was too prone to aquaplaning.
Haas driver Romain Grosjean said there had been "a lot of progress made from last year" but said the 'intermediate' tyre "got destroyed a bit too early, so some work is to be done".
Steep learning curve for Stroll
Stroll, who is making his debut in F1 this season, is bringing considerable financial backing to Williams - said to be at least £20m this year.
That is in addition to a similar amount last year, when the team provided him with an extensive test programme in a two-year-old car at circuits around the world to prepare him for his debut.
But he has had a torrid introduction to F1 in Spain this week.
On Tuesday, his first day in the 2017 Williams car, Stroll crashed at the high-speed Turn Nine after just 12 laps and damaged the front wing.
Williams had to stop running for the day while the wing was sent back to their base in Oxfordshire for repairs and a new one was flown out for Wednesday.
Stroll then completed 100 laps before crashing again at Turn Six.
Williams engineering chief Rob Smedley said Stroll had been caught out by the lack of grip on cold tyres.
"Lance was out on cold tyres, on an out-lap with a lot of fuel in the car, and the tyre stepped away from him," Smedley said.
"He was an innocent victim of that happening and what should have been an innocuous sideways moment brought him around into the barrier and did some damage. That happens and we expect it to happen, there is no blame on his part."
The second and final pre-season test runs from 7-10 March, with the first race of the season in Australia on 24-26 March.
Fastest times on day four of testing
1 Kimi Raikkonen (Fin) Ferrari 1:20.872 *
2 Max Verstappen (Ned) Red Bull 1:21.769 *
3 Jolyon Palmer (GB) Renault 1:21.778 *
4 Romain Grosjean (Fra) Haas 1:22.309 **
5 Antonio Giovinazzi (Ita) Sauber 1:22.401 ***
6 Sergio Perez (Mex) Force India 1:22.534 **
7 Stoffel Vandoorne (Bel) McLaren 1:22.576 ***
8 Valtteri Bottas (Fin) Mercedes 1:23.443 *
9 Nico Hulkenberg (Ger) Renault 1:24.974 *
10 Daniil Kvyat (Rus) Toro Rosso no time
Toro Rosso's Carlos Sainz, Williams' Felipe Massa and Mercedes' Lewis Hamilton did not run.
* time set on soft tyres;
** time set on super-soft tyres
*** ultra-soft tyres