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.
Then just limit to 4 players but allow up to two to recruit a companion, that issue is solved?
No it really doesn't because then they would basically have to rework all the encounter mechanics for it.
Was talking about the issue of coop limited to 4 players.
Balancing they would only need to rework the second half of act 1, the part we can't reach yet. It would make what we have right now easier but that isn't too much an issue as long as later stuff is made with 6 characters in mind