I personally to migrate from xCode 3 to 4 is pretty simple, just open the project with xCode 4 and everything should work, if you also migrated to version 1.8, you should add the library libstdc++.6.0.9.dylib that worked for me.
Solved! I managed to add the libstdc++.6.0.9.dylib and the CoreText into my project framework. Succesful migration to XCode 4 + API v1.8
Alberto