Multiple PCs, one monitor
These quad screen splitters are pretty expensive:
SmartVM
Network Technologies
I think a viable solution would be to run 4 instances of Remote Desktop Connection - this would definitely work in the XP case, and appears to be supported in Windows 7 as well. If you have a good-sized monitor, you can tile the sessions and have full interactivity with all of them. You can also zoom into one for a short period of time if needed.