Just for the record, you can only upgrade to CS6 from CS5 now.
The upgrade window for earlier versions (CS4 and older) closed on Dec 31, 2012. So CS4 to CS6 is no longer an option.
Have you tried to install CS4 on Win 8 anyway to see if it works?
Indeed... The single way to "upgrade" is to pay full price (about $2k).
I was intertested in upgrading to Win 8 and going through the Win Upgrade Assistant it alerted that my CS4 is not compatible. So I am trying to find out options before I go that path... I think if I start the upgrade to Win 8 I can't go back to Win 7 if I have problems with CS4... so I'll get really stuck then!