Where is the Xcode installer located?

You could try inspecting the output of

$ pkgutil --pkgs

to see if any of the names look like the package that claims to be installed, then

$ pkgutil --pkg-info thatpackage

to see where it was installed. If it's not actually found there, you could try

$ sudo pkgutil --forget thatpackage

and see if the App Store will let you re-download; but at that point I'd probably contact App Store support first, as mucking with the package database might cause bigger problems (like, conceivably, forgetting that you ever bought the program — although I would hope Apple's programmers were smarter than that).

FYI, I had a similar problem but eventually noticed (via spotlight search for 'xcode') that rather than installing the actual app, it had downloaded an app in /Applications called, "Install XCode" which was 4.58 GB.

Running that does the actual install of XCode 4.