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. :-/
//edit: Personaly i believe that give us option to allow another two members in single-player game only would be quite easy ... but for some unknown reason it seems like Larian simply demands to keep the game exactly the same in both options.