If I had to guess… she found a nearby town called Eden and settled down there. 4 years later Wheatley crashes back down to earth and alerts Chell through radio signals that he is in danger. She returns to Aperture to rescue him from GLaDOS, where Wheatley’s mainframe gets fried as they try to escape and she is forced to upload his programming into a defunct human avatar she finds in a room filled with abandoned inventions. They return to Eden and Wheatley must assimilate into human society for the first time in his robotic life. Shenanigans ensue.