If you're going to break up, break up for a while. Don't shut things down and screw over your fans one day and then, a scant four years later, pretend nothing happened.
When a band breaks up, it should stay broken up for at least a decade or more. Breaking up and reuniting within five years is a hiatus, not a breakup.
Why does anyone care about Fall Out Boy? Who wants to stay a lame kid their whole lives? This is a pointless band that makes pointless music and doesn't stand for anything other than the perpetual lameness of emo and commercialism.