With Toribash 3.8, you can not play 3/4 player fights.
You need to have 3.7x or older.
If/When you have Toribash 3.7x or older, you need to use the 3/4 Player Match Generator in order to get 3/4 player matches available in Single Player.
That's right, Single Player. It's not available in Multiplayer, and probably won't ever be.
gl