Did you install XCode 4 via App Store, rather than the Dev account? You may be one of quite a few people that see strange behaviour when they want to update their XCode (after seeing in their App Store purchase section, an update is available). Well when you try and update, it spits out a strange error message, like: You have updates available for other accounts
Very strange behaviour from Apple and certainly a glitch, or bug as it won't let
you update. In fact, you wouldn't have known there was an update unless you went to the purchases section, whereas for other apps, you get a badge notification saying you have a new update, right when you launch the App Store.
Well, a simple fix would be to go into your Applications folder and delete the Install Xcode.app file that gets downloaded first when you install XCode.
Then you should be able to re-load and update XCode 4. Hopefully we won't have to do this for too many updates, and Apple get their act together and fix this.