They presumably get back together in Five Stars for Beezus.
In the first episode, 'Pilot ', Jess moves in with Nick, Schmidt, and Coach.
She doesn't like Caroline and tries to later in the desert after Nick freaks out, Jess tells him she wants him to be happy, even if that means him moving in with Caroline, and that she'll be fine because she met him. He then goes home to Caroline the next day, however, that night, Jess sees the moving van from her window, opens her door and sees Nick standing by his door.
They smile at each other and Jess is glad he didn't move out.
In Elaine's Big Day start dating and in Mars Landing break up.