Does the multiplayer have to be 4 players? Can I play as a team of 2? Does AI round-out the rest of the team? 

Yes, it can be 1, 2, 3, or 4!

AI bots can be added to fill out the team, or the player can try with just the number of humans available. 

There is also join in progress, so one player can start with 3 bots, and get them replaced by human players throughout the mission.