I believe the reason for them sticking to 4 characters is that their engine probably can't handle more than 4 players at once in co-op.
And dont forget that there is split screen multiplayer planned ... There isnt many games that would divide screen to more than 4 segments.
But, +1 agreed with OP, it would be nice.
In the words of the senior NCO instructor at cadet battalion: “If you ain’t cheating you ain’t trying. And if you got caught you didn’t try hard enough!”
