It's a big trade-off when it comes to four players (actually everything is a trade off when you build panels). It's going to be wider than you may like it to be but they'll be plenty of room or you can make it compact and let everyone cramp together like an unwelcome game of twister. Check out some of the cabs on KLOV to get some ideas of what 4-player cabs look like (ignore Gauntlet though - I always thought that was a horribly designed panel - my opinion at least). Also the examples page has a lot of good 4-player cabs that don't look too wide but still fit four comfortably.
As for the speakers - someone else may be able to help you better with that. If you're not using it as a jukebox though I would say, once again my opinion, that simpler speakers would be fine.