@Martin, don't rename old one, that gave me problems, what it worked for me was to install XCode 4 in a separate folder.
Only few issues, if you have both opened at the same time, be sure you close the simulator every time you switch to one to another, once the XCode 3 simulator is opened, XCode 4 tries to use it and it won't work, and same the other way round.
At some point the Organizer also went a bit nuts, but I closed and re-opened and all was fixed.
