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.