F1 testing: Valtteri Bottas fastest on final day
Williams driver Valtteri Bottas set the pace, as Mercedes emerged from the final day of pre-season testing as strong world title favourites.
Mercedes driver Nico Rosberg was sixth, but the team have done enough over the winter to prove they are still well ahead of the rest of the field.
Bottas was 0.406 seconds quicker than Ferrari's Sebastian Vettel, both using the fastest tyres, the super-soft.
McLaren had another troubled day, Jenson Button managing only 30 laps.
The team, starting a new engine partnership with Honda, have done less pre-season mileage than any team other than Force India, whose car appeared for the first time only on Friday.
Button was consigned to the garage for the morning session after the team discovered a problem that required the engine to be removed.
The team then had a stuttering afternoon, at the end of which Button was eighth fastest, 2.264secs off the pace.
"I am pleased with the winter," said Rosberg. "We have done a good job with the development of the car. That's not to say the others aren't also quick but I am optimistic for the first couple of races because we have a good car.
"I am comfortable, setting the best time of the whole winter testing two days ago."
Sauber's Felipe Nasr was third quickest ahead of Red Bull's Daniel Ricciardo, Toro Rosso's Max Verstappen and Force India's Sergio Perez.
Lotus driver Pastor Maldonado had a light crash at Turn Four just after lunch and was slowest of all.
Lotus said the crash was caused by a "braking-related issue" which meant Maldonado could not slow sufficiently for the corner.
The season starts with the Australian Grand Prix on 13-15 March - live coverage of the race is on 5 live and the Sport website and app, with highlights on BBC TV.
Final pre-season test. Day Four, Circuit de Catalunya
1. Valtteri Bottas (Fin) Williams-Mercedes 1:23.063
2. Sebastian Vettel (Ger) Ferrari 1:23.469
3. Felipe Nasr (Bra) Sauber-Ferrari 1:24.023
4. Max Verstappen (Ned) Toro Rosso-Renault 1:24.527
5. Daniel Ricciardo (Aus) Red Bull-Renault 1:24.638
6. Sergio Perez (Mex) Force India-Mercedes 1:25.113
7. Nico Rosberg (Ger) Mercedes 1:25.186
8. Jenson Button (GB) McLaren-Honda 1:25.994
9. Pastor Maldonado (Ven) Lotus-Mercedes 1:28.272