Ben is correct there should not be any major issues with running a WS 4.x VM , GSX3.x, or ESX2.x vm as long as you just want to power it on or off and have basic functionality. Just be sure that if you are going to move them from one host to another that you always do it with a fully powered down VM and not a suspended one.
Also you may not want to upgrade the tools in the older VMs if you plan to move them back to the older platform there is a risk of hitting some sort of backwards compatibility issues
