You can have(and I would recommend leaving) CS4 and CS5 on your PC at the same time. Only one version can be opened at any one time. I have always kept the previous version on until I made sure the newest version works as it should and to make sure any third party plug ins I have are compatible in the new version or until upgrades/updates are available. When you are satisfied that all is well, you can "deactivate" then install the previous version. Make sure you deactivate before you uninstall.
I was considering leaving CS4 and CS5 both on my PC, so I can use the numerous Plug-Ins with CS4, and the advanced features of CS5. Content Aware fill is something I will use often. I know I can reinstall the plugins in CS5, but it is always a hassle, some I don't remember where I downloaded to, some I have original disks, then upgraded on-line etc, and a million passwords to retrieve them on line.
Is this viable?