If you use the null output plugin you can run multiple copies of winamp on one machine.
This is specifically for shoutcast streaming. You have to go through the preferences and tell winamp to allow multiple instances. One copy can play throught the sound card, the rest need to run to the null output.
You can set up a multi sound card box, but personally I'd prefer not to have 4 sets of speakers playing different things.
There are plugins that allow you to do interesting DJ effects, I think there is one that allows you to mix.