Mercedes driver Valtteri Bottas believes Ferrari are "a bit ahead" of the world champions at the end of the first week of pre-season testing.
Ferrari have consistently set fast times over the four days at the Circuit de Barcelona-Catalunya, while Mercedes have taken longer to get up to speed.
Bottas said: "They do seem very strong. No matter which fuel loads or engine modes they're running, they are quick."
Renault's Nico Hulkenberg was fastest on the final day.
The German was split from team-mate Daniel Ricciardo in third by Toro Rosso's British-born Thai rookie Alexander Albon, with Bottas fourth.
There are four more days of testing next week before the first race of the season in Australia on 14-16 March.
On Wednesday, Bottas' team-mate Lewis Hamilton said Ferrari were "very strong", adding: "It appears they have a better package than last year, which means it will be a bigger challenge for us."
The world champion and Bottas finally set some quick times on Thursday, and both posted a faster lap than Ferrari's Charles Leclerc by lunchtime on the final day of the test. But they used softer tyres to do it - and the Ferrari's underlying pace appeared clear.
"I think we feel at this point they're going to be a bit ahead," said Bottas.
But Leclerc said he believed their rivals may be "sandbagging" - hiding their true pace - at this test.
"The performance has no sense for now because it remains testing," he said. "They are probably not pushing and we are not either. We don't know how much the others are sandbagging so we'll see at the first race."
Bottas, meanwhile, gave an insight into the personal battles he had faced over the winter in coming to terms with his 2018 season, in which he failed to win a race while Hamilton took 11 and the world championship.
He said: "It was a disappointing season and it kind of nearly made me angry with myself that in my six years in Formula 1 I haven't been achieving my target.
"Obviously, I have time yet, but I'm starting to realise you definitively only have one career, and I don't want to be in that situation again, that I feel like that and I start to question myself if I could have done something better.
"I just want to make sure that this year I'll do everything I can to achieve those targets and I'm willing to do whatever it takes to make sure I achieve those goals.
"I'm just mentally prepared for the whole year, but it all comes down to details in this sport.
"I just have to be the best that I can. I know what I can do if I perform at my best level - I'll be able to be matching and being ahead of Lewis in qualifying or race pace. I know I can do it and now it's just hard work to be up there more consistently.
"I didn't win a single race last year, and there is only one target for the whole season - the world championship, for me personally and for us as a team."
Renault on top
Hulkenberg set his fastest time on the softest tyres - which were used by all of the top four - before stopping out on track late in the day.
Bottas and Hamilton, in fifth on the second-softest tyres, appeared towards the top of the times for the first time all week, ahead of Leclerc and British rookie Lando Norris in the McLaren.
Williams, whose car only made it on to the track on Wednesday afternoon - midway through the third day of the test, concentrated on learning their car and drivers George Russell, another Briton making his debut this season, and Robert Kubica propped up the times.
The second and final four-day pre-season test begins on Tuesday 26 February and runs until 1 March.
Fastest times, day four, first pre-season test, Circuit de Barcelona-Catalunya
1 Nico Hulkenberg (Ger) Renault 1:17.393 *****
2 Alexander Albon (Tha) Toro Rosso 1:17.637 *****
3 Daniel Ricciardo (Aus) Renault 1:17.785 *****
4 Valtteri Bottas (Fin) Mercedes 1:17.857 *****
5 Lewis Hamilton (GB) Mercedes 1:17.977 ****
6 Charles Leclerc (Mon) Ferrari 1:18.046 ***
7 Lando Norris (GB) McLaren 1:18.431 ****
8 Antonio Giovinazzi (Ita) Alfa Romeo 1:18.511 ***
8 Romain Grosjean (Fra) Haas 1:18.563 ****
9 Kevin Magnussen (Den) Haas 1:18.720 ***
10 Pierre Gasly (Fra) Red Bull 1:18.780 ***
12 Lance Stroll (Can) Racing Point 1:19.664 **
13 George Russell (GB) Williams 1:20.997 ***
14 Robert Kubica (Pol) Williams 1:21.542 **
Key: * = C1 tyre (hardest); ** = C2 (second hardest); *** = C3 (middle); C4 **** = (second softest); ***** = C5 (softest)