So we can include a lot of playable characters for the 3 joining players to select from (1 is always static in the team leader slot) but there is either a separate function hiding away in the many data files or a built in one in the game engine to actually change the 4 player limit of co-op servers.

Of course it is possible from what I've seen so far to just build a Multiplayer Adversarial map in the same way you would an SP mission.... just save it differently, but adversarial has automatic Island limitations and other problems that might get in the way during a mission.

Any thoughts?

The 4 players must be in the same Fireteam, maybe modders will figure how to change this but for now we are out of luck.

What is possible to do however is to control more than 1 Fireteam by either attaching another Echelon to the player's or to have another Echelon following the player's echelon.

I've had 1 Echelon with several fire teams attached, all with playable men, players can then choose from any of the men from any fireteam, apart from player 1. No matter what man you choose you are still being ordered around by player 1 though.