One of the major changes for Xcode 4 is that Apple is moving from gcc to llvm as the primary compiler. They still include gcc in the package as of now, but it

One of the major changes for Xcode 4 is that Apple is moving from gcc to llvm as the primary compiler. They still include gcc in the package as of now, but it is based off a now-old gcc version (4.2 series, back when it was GPLv2) for which they have already released their source changes and it is really only there so old code that expects gcc specific features can continue to be compiled... it is basically deprecated at this point.