Randomly, I'm already managing 8 "characters" cause by the end everyone has a familiar in my games. Shadowheart needing a feat of course. So the engine is shown both in the prologue and with summoning to handle way more than four. Add in the zombie glitch (take your pick of which one) and the engine can support an immense amount of player controlled entities without faltering, glitching, or failing in any way. Essentially, the game can totally support 6 or any number within reason, but 4 ended up being chosen.
That is all nice, but it aply to single player only ... I belive that multiplayer is the real issue here, since Larian promised people split screen, it would be ridiculous to split screen in 8 squares. :-/
Simple. Splitscreen goes up to four players local for that fun couch action, with it also supporting two more over lan or internet. Normal internet/Lan/etc play allows up to 6 players. Both allow 6 characters. My whole thing was on characters not players there, I don't think the number of characters hinders splitscreen in any way and having one console only support 4 players but also allowing 2 others to connect over the internet makes sense to me. And honestly I feel like splitscreen co-op is a dying thing but keeping it afloat isn't a bad thing.