I want to remove all existing SDK versions as well as Xcode 4.
Xcode 4 Guide says to do this:
sudo /Xcode4/Library/uninstall-devtools --mode=all
sudo /[xcode-path]/Library/uninstall-devtools --mode=all
Normally, [xcode-path] means /Developer, but if you have multiple versions, for example 3 is the first installed, 4 is second, /Developer will be xcode 3's root derectory and /Xcode4 for xcode 4.