I recently spent a bit of time installing Macports GCC 4.6 and modifying some Xcode compiler templates so I could select and use it from my Xcode projects. I did eventually get it working, the main drawback was that GCC 4.6 doesn't know what to do with "blocks". If you are planning on doing any COCOA, GUI stuff then this probably isn't a good option for you (as a bunch of the NS Object header files contain blocks, you get stopped pretty early).
If you are doing any other kind of project you can probably get it to work with this method.
You can even get it to create universal binaries with some creativity. I have all the gory details on how I accomplished this here:
http://thecoderslife.blogspot.com/2015/07/building-with-gcc-46-and-xcode-4.html