If they create the game with 4 party members in mind thats all fine and dandy. But there has to be mod support for having larger parties. I prefer the larger party sizes myself and if I were to guess the reason why they limit to 4 members is that its alot easier to balance. Another effect you get is that in co-op mode you only have to wait for 3 players and not 5 til its your turn (we all know that certain player who would carefully consider every tiny action for 5 minutes before acting, only for your turn taking 10 seconds).
