Most console games that support splitscreen support four players these days, so I don't see why not. But people want offline multiplayer to die. But what if you can't connect to Internet? What if you have a meeting in your house and people want to play SRB2, but there's only one computer? There are lots of reasons for offline multiplaying :P
Anyways, more than two players on the same keyboard is painful and with new hardware simply it won't work (there's even trouble with several combinations with a single player, think on it). Multiple joysticks become a must.
Also, the split screen for four players isn't all vertically, it's two vertically and two horizontally, for those who don't know :P
EDIT: and shut up Chaos, SRB2 runs at full speed here even in software mode at 640x400 :/ There's no excuse for it.