The start of the game involves breaking out of prison with your chosen companions. There is no tavern where sourcerers hang out where you can recruit another if a companion leaves.
Also, the story is being written in such a way that there are always reasons for the characters to work together for the main story, despite any competitive questing which may happen along the way in multiplayer. That story exists for the companions in single player, as well.

Personally, I liked role playing both main characters in D:OS, but with 4 characters I'm not sure I'd want to do that, even with that option left in.