barab: at the end of season 3 the "good" aliens said they are going to relocate all humans to brazil, and fight the bad aliens. tom & co decide they want to keep fighting so they are going to leave and head back to charleston, NC (they are in boston). season 4 starts with them outside the charleston city limits, where they get separated into 4 groups (rebel ann, hippies with lexi, nazi youth camp, and adults in the alien ghetto) and the show basically jumps forward a few months.
oh the good aliens also leave cuz reasons, like 10 of them total have returned. i guess its a plot line the writers are trying to weasel out of