I recon you could buy splitters for the video signals of your four boxes and then connect one of the signals per CPU to the decicated screens while connecting the other to a regular KVM switch. You can have one PS/2 keyboard and mouse and as many USB keyboards and mice per system as you want, so using a regular PS/2 KVM switch and USB kb/mouse on the units directly or the other way around but using a USB switch should both work.

I am still non-plussed regarding why you would want to do this. Simply because as soon as the spare display/keyboard/mouse is controlling one CPU while you have someone else work on the box there is a conflict. Since there is the conflict then, I gotta wonder why you dont use the CPU directly. =]

If you want to have all four installations available on demand, you are better served creating virtual machines of each system and putting those on the a separate CPU on the spare. That way you could use five systems at the same time with one being the wildcard/joker system with all the programs of each computer being available in turn.

Hope this helps.