Is there any way to add more than two teams to play as, so that four players could be on separate teams?

I'm not sure, but I would sure as heck like there to be.

And can you have 2 players on one team fight two enemies on AI?

Currently the teams are hardcoded to work like this:

1-4 human vs. cpu
1 human vs. 1-3 human
2 human vs. 1-2 human
3 human vs. 1 human
cpu vs. cpu (by way of fancy scene/activities coding, not by team selection menu)

You can only have two human teams total. Technically, there is a "third" team, which is used by the crab in the tutorial scene. The third team attacks both of the other teams.

Team = 0 is Red team.
Team = 1 is Green team.
Team = anything else (-1,2,3,etc.) is the "wildlife" team.

...of course, we have no idea what will be possible in newer builds. Who knows, maybe we could add more teams via Lua.

Wildlife team? sounds like some sort of group of savages.

Especially how they attack both teams.

It's for the animals, learn to read.

I know! It was just a joke.

Jeez, do I really need to slap smilies onto everything for people to understand me?