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.jpg

Of course the last player to join will have an interesting character creation screen :

http://imgur.com/jRC5rwp.jpg

And 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.png


PS. 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!