Nico Rosberg topped the final morning of preseason Formula 1 testing as Mercedes continued to set the pace at Barcelona.
After teammate Lewis Hamilton ended up fastest on day three with a 1m20.558s, Rosberg led the way early on day four. His best of 1m21.277s remained quickest for some time before the German and Fernando Alonso''s Ferrari started a sequence of short runs on the soft-compound Pirelli rubber.
Just after the session's halfway mark, Alonso managed a 1m21.024s lap, but almost immediately the Mercedes recorded 1m20.723s.
Alonso moved the mark to 1m20.606s in the final hour, only for Rosberg to make sure of the fastest time with 1m20.130s. He later set a 1m20.180s and has thus produced the two best laps of 2013 Barcelona testing so far.
Alonso eventually managed a 1m20.494s before switching to the super-soft, on which he failed to improve.
Almost a second behind the Ferrari came Jenson Button's McLaren. He spent the early part of the session on harder rubber, but had a late run on unmarked Pirellis (suspected to be the soft compound) to end up third.
The midfield battle was close, with a tenth covering the Force India, Sauber and Williams times at one stage.
Sauber''s Nico Hulkenberg ultimately pipped Paul di Resta's Force India for fourth, with Pastor Maldonado completing the top six. Sebastian Vettel concentrated on the harder rubber compounds and was only seventh fastest.
Gearbox problems struck Kimi Raikkonen on his installation lap and his Lotus only made it back out in the closing moments of the session.
Pos Driver Team Time Gap Laps
1. Nico Rosberg Mercedes 1m20.130s 53
2. Fernando Alonso Ferrari 1m20.494s + 0.364s 64
3. Jenson Button McLaren 1m21.444s + 1.314s 44
4. Nico Hulkenberg Sauber 1m21.541s + 1.411s 46
5. Paul di Resta Force India 1m21.664s + 1.534s 38
6. Pastor Maldonado Williams 1m22.415s + 2.285s 42
7. Sebastian Vettel Red Bull 1m22.514s + 2.384s 59
8. Charles Pic Caterham 1m23.115s + 2.985s 53
9. Jules Bianchi Marussia 1m23.167s + 3.037s 62
10. Daniel Ricciardo Toro Rosso 1m23.628s + 3.498s 46
11. Kimi Raikkonen Lotus 1m24.396s + 4.266s 7