Season four witnessed the return of Peter from "inexistence", but after having used the machine to save both universes, he came home to find his timeline had been irrevocably altered. No one remembered him; no one had had his presence touch their lives. It wasn't until 4x12 "Welcome to Westfield" that Olivia finally began to regain her old timeline memories, because of the cortexiphan Alt-Nina had been secretly dosing her with.
I included in this montage every kiss (and almost kiss!) between Peter and Olivia during season 4. BRIGHT BLUE means the kiss is occuring inside a dream, while WHITE stands for reality.
I absolutely love the last scene. Guh. Perfect season finale.