One of the Devastation bosses supports a 4v1, but that's all I can think of.
Oh, I see it now. I've been looking in the "modes" chapter but you have to look in the setup of each boss to know how many fighters can team up.
Maybe a 2v3 where the 2 side has unboosted Ex characters (30 Life, +0/1/1 on Unique, Ex UA) would work?
Or maybe trying with some items from the Armory or tinker a little bit with the Strikers (I haven't played with them yet so I don't know if that can be used for balancing uneven teams).