A possible solution to Xcode 4 freezing after you open a project:

If your Xcode project source path contains a space then rename it and try opening the project again. This is the most likely cause.

If it still freezes after renaming then do the following:

1. (Force) Close Xcode
2. Delete these cache files:

rm ~/Library/Caches/com.apple.dt.Xcode
rm ~/Library/Preferences/com.apple.Xcode.plist

3. Restart Xcode & keep your fingers crossed!