The fourth and penultimate day of private Formula 1 tests ahead of the 2026 season left the main highlight as the solid work of Mercedes, the track debut of Aston Martin, and a challenging day for McLaren at the Barcelona circuit. Mercedes was once again the team with the heaviest workload. Their car showed aerodynamic solutions different from their rivals, and it is expected that Fernando Alonso will drive it on the final day. McLaren, in turn, had to cut its program. With this session, Mercedes used up its three permitted testing days in Barcelona and left as the team with the most laps completed, with reliable performance and positive evaluations from its drivers. In contrast, Aston Martin had a minimal participation. Liam Lawson also drove for Racing Bulls, and Sergio Pérez for Cadillac. The AMR26, still without livery and designed under Adrian Newey's direction, only hit the track in the last hour of the day, with Lance Stroll at the wheel. The Canadian completed five laps before causing a red flag by stopping on track near the end of the session. Andrea Kimi Antonelli and George Russell alternated at the wheel of the W17, completing a mix of long runs and shorter simulations focused on different power unit modes. Russell set the unofficial best time of the day at 1m16.6s, although the team prioritized data collection over lap times.
Oscar Piastri took over the MCL40 and completed 48 laps before being sidelined by a fuel system problem, forcing the team to dismantle the car for analysis.
