It’s ok to delete anything in there.
It is totally possible that there could have been 4GB of stuff in there. However, there may have been other types of temp files littered in other parts of your system. Also, Windows XP keeps System Restore points for a period of time to allow you to restore your system should something newly installed cause your system to fail. After a certain period of time these points are automatically purged freeing up new space.
Well, at least you now have your space back. 
