So, you get the latest generation of Mac which only support Lion and above, which only runs Xcode 4 or later, which is great. You have few old projects that use 10.4 sdk, which only compatible with gcc 4.0. You don’t want touch those project but want to be able to build them just as before. Here we are, copy few files and folders from your earlier Xcode instation into the /Applications/Xcode.app, you can build them as before.
/Applications/Xcode/contents/Developer/usr/lib/gcc (create the lib directory)
/Developer/usr/bin/*4.0* (any file name contains 4.0, about 9 of them)