Sorry, I do not plan to add a four players mode. One big reason is that it would limit the possibilities for characters graphics: currently each character can take up to almost half of the video memory which is useful as they have more than 20 animations each.
So until I find a clever way to avoid sacrificing too much of the animations, it is advised to run a tournament when playing with more than two players. There even is printable themed brackets here: http://supertiltbro.wontfix.it/super-tilt-bro-images/miscs/brackets.png
