Mac doesn't like you to open multiple copies of an application. If you try to open a new copy, it will switch to the old copy. If you want to open a new copy this is what you need to do: Fire up the terminal and type: $ open -n...

To uninstall XCode 4 $ sudo /Xcode4/Library/uninstall-devtools --mode=all To uninstall XCode 3.x.x $ sudo /Developer/Library/uninstall-devtools –mode=all