My retail copy of LR 4 which I've using since 4/12 won't open. The msg
"This Beta version has expired. The app will now quit" appears.
You were using the 4.1 release candidate, by the sounds of it.
Just download the 4.1 final version and you should be all set.
Do I need to uninstall lightroom first?
Probably a good idea to uninstall first.
No you do not need to uninstall. Just download the appropriate version for your system as posted by Victoria, then run the installer.
If it was a beta version, it is best to uninstall first
Europe, Middle East and Africa