I think your view of the 4GB download is going to depend on your Internet speed. At 1 MB/s or so, I've found it annoying to wait an hour for a download, but it's hardly the end of the world for an occasional thing. Could definitely be improved.
What's wrong with this at 69MB?
http://llvm.org/releases/2.9/clang+llvm-2.9-x86_64-apple-dar...
Your point makes sense if you accept a 4GB download just for a compiler.
There are many Linux distro's that come with a C-compiler which are less than 4GB in total size.