This achievement requires you to be part of a squad of 4 players (coop) in which every individual Dead Island character (Logan, Xian, Purna, Sam B) is present.
This sounds easy, but from what I've learned this game is very buggy (even more than the original 360 version when it comes to coop).
Sometimes it can take a while until everyone is able to join your game. If you try this with randoms, there is only a small chance of you A) finding a group of 3 players and B) every individual character being present there.