The Scuderia's new SF15-T had already topped the first and second days in the hands of Sebastian Vettel, and Raikkonen carried on that momentum by setting the best time of the test on Wednesday, dipping down to 1m 20.841s to finish comfortably clear of the chasing pack.
"Over the winter, we have made a good step forward and this car is definitely an improvement over the 2014 one," Raikkonen reflected. "It's the whole package that has progressed, but there is still a lot of work to do.
"We are not comparing ourselves to the others, we just got on with our own job, without looking at what they were doing. Times are of relatively little importance; what matters is that we have put together a good number of laps without having any real problems. We now have a good basis from which to start the development work."
Behind him, Marcus Ericsson continued Sauber's encouraging start to the new season by finishing second fastest, after new team mate Felipe Nasr had put the team top of the timesheets on day three.
The Swede also broke into triple figures in the lap charts, although Mercedes once again led the way in terms of total mileage as Lewis Hamilton clocked up 117 laps en route to setting the third fastest time of the day.
Ericsson said Sauber's development over the four days had been particularly encouraging, adding: "It's been a great first week of testing for the whole team - we collected a lot of good information. It was interesting for me because I drove the car on days one and four, and it was obvious that we were able to make some progress in between."
Max Verstappen worked his way up to fourth for Toro Rosso, as the team continued to clock up far more mileage than parent team Red Bull, with Daniil Kvyat restricted to 64 laps of running in the new RB11.
"Our focus has been entirely on reliability, while allowing the drivers to experience every aspect of the job," Toro Rosso technical director James Key said. "We haven't done any work on performance so far and we will not do that until the end of the month, therefore we are not looking at our lap times for now. A good start and we must continue like this in the next tests."
Kvyat's tally was still more than McLaren managed, as Jenson Button was disrupted by engine problems in the morning and then a fuel pump fault in the afternoon. However, in between issues the Englishman managed to help the Woking team to their most productive day of the test, completing 35 laps and setting a best time of 1m 27.660s.
McLaren engineering director Matt Morris said despite some niggles the team leave the first test feeling positive. "Our prime objective coming away from this test was to ascertain that the main car architecture worked, and we’ve done that," he explained.
“Today, we had a couple of stoppages, but we were still able to get on with our aero programme - obtaining correlation data for the guys back at the factory, just so we can check that everything’s working as we expected.
“Additionally, Jenson’s been able to give Honda some really good feedback - he’s been constantly providing analysis after every run. That’s to be expected: it’s a new power unit, so there’s a lot of information that being sent back to Japan, and Honda are constantly upgrading the car.
“We head to Barcelona hopeful of further refining and developing the package.”
Button ended the day eighth overall, behind Verstappen, Williams' Felipe Massa, Lotus's Romain Grosjean and Kvyat in the Red Bull.
"A slightly frustrating morning for us, with an ERS problem," said Red Bull head of race engineering Guillaume Rocquelin. "It cost us a few hours, but to be honest we probably didn't lose too much meaningful running. This week has all been about establishing reliability and while we've had a few niggles the car is pretty much where we want it to be."
Three red flags punctuated the day - Kvyat brought the first out after that ERS issue after just eight laps, while Hamilton spun at the chicane at midday and then Ericsson stopped out on track ten minutes from the chequered flag.
The teams now have two weeks before testing resumes at Barcelona on February 19-22. Force India are expected to join the fray, although their new car is not likely to appear until the third and final test, also at Barcelona, on February 26-March 1.
Unofficial Wednesday testing times from Jerez
1. Kimi Raikkonen, Ferrari, 1m 20.841s, 106 laps
2. Marcus Ericsson, Sauber, 1m 22.019s, 112 laps
3. Lewis Hamilton, Mercedes, 1m 22.172s, 117 laps
4. Max Verstappen, Toro Rosso, 1m 22.553s, 97 laps
5. Felipe Massa, Williams, 1m 23.116s, 73 laps
6. Romain Grosjean, Lotus, 1m 23.802s, 53 laps
7. Daniil Kvyat, Red Bull, 1m 23.975s, 64 laps
8. Jenson Button, McLaren, 1m 27.660s, 35 laps