This is probably a known issue considering you can see it from the game lobby screen. But you can have 5 players join the same game now :
http://imgur.com/1xEmvA2.jpgOf course the last player to join will have an interesting character creation screen :
http://imgur.com/jRC5rwp.jpgAnd will steal the 4th player's character to play with once in-game, and give him (the 4th player) a black screen to let him know he's been screwed :
http://imgur.com/TgH3Vo2.pngPS. I tried all this by opening multiple instances of the game on my computer. The load times with 5 games open is surprisingly very short, nice optimization!