How about a back-to-back (head to head, really) setup - a 2-player CP with monitor and a mirrored setup on the other side (with both monitors running off a splitter on the computer output) 180degrees out of phase, in other words. I'd not be conventionally space friendly, but could work like a kiosk in the middle of a game room.
You could even rig two systems in it, having it run as 2 stand alone rigs - but then flip a couple of switches (usb switch for the KB encoder and a switch for the PC output) to change over to a head-to-head 4 player setup.
I don't know, that just popped into my little brain...