The 23-year-old Frenchman beat FDJ duo Anthony Roux and Arthur Vichot to the line after an 11-man escape group held off the peloton to contest the stage.
Björn Leukemans and Bert-Jan Lindeman (Vacansoleil-DCM), Pierrick Fedrigo (FDJ), Cyril Gautier (Europcar), Romain Bardet and Hubert Dupont (AG2R-La Mondiale), Jonathan Hivert (Sojasun) and Julien Antomarchi (La Pomme Marseille) were the other escapees on the 152.6km stage around Besseges.
Three ascents of the first category Col de Trelys saw an action-packed stage with Gautier, Dupont, Hivert, Lindeman, Antomarchi and Vichot joined by Jurgen Van de Walle (Lotto-Belisol) and Georg Preidler (Argos-Shimano) in an early escape that held a lead of over two minutes.
That gap was down to 30 seconds with 40km to go when Cousin, Roux, Leukemans, Fedrigo and Bardet bridged the gap and with 10km left the remaining 11 leaders held a 1:10 over the Topsport Vlaanderen-Baloise- and Big Mat-Auber 93-led peloton.
Cousin now takes the overall race lead from stage one winner Michael Van Staeyen (Topsport Vlaanderen-Baloise) who led the peloton over the line in 12th place.
Stage four of the early-season five-day race held annually in Southern France sees the riders travel 154km from Sabran to Pont Saint Esprit.