Franck Haise's Lens, only in their second season back in the French top flight, were looking to move back within 10 points of league leaders Paris Saint-Germain. But a nightmare first half put paid to any hopes of that, as Steve Mounie gave Brest a third-minute lead before goals from Brendan Chardonnet and the in-form Romain Faivre made it three at the break.
Lens' miserable afternoon was completed in the second half as Jeremy Le Douaron added a fourth for the hosts in the 69th minute and the away side saw substitute Arnaud Kalimuendo sent off.
The result leaves Rennes in second, 12 points behind PSG, after extending their unbeaten run to 11 matches in all competitions with a 2-0 win over Montpellier on Saturday.
Lionel Messi scored his first Ligue 1 goal as PSG stretched further clear with a 3-1 victory against Nantes.
Nice, who visit Clermont, and Marseille, who take on Lyon, can both leapfrog Lens later on Sunday.
Standout matches today