4 desktops?? There are some softwares that run on Aero that let you change your desktops (scroll through them sort of like a smartphone)
if you're talking about having 4 truely seperate machines...
I use 'VMware' at work. I believe they have a stripped down version that is free to use (or at least trial version). There are some freeware similar products, but they're not as good. VMware.com