I can't speak to Apache or mod_perl but I have had to deal with cross_version compatibility issues with GCC and other modules.

Of course the GCC developers try for backwards compatability and many modules compile fine with any version of the compilers. Compiler optimization are, by nature, machine dependant and often an optimization that works on one platform will break another platform. I've been working with the ATLAS libraries. These use a very agressive optimzation strategy. Turns out the strategy has been shown to only works with GCC 2.4. When I upgraded my OS my GCC got upgraded to 3.4.2 which broke ATLAS for me:-{

Many modules should see little difference between GCC versions but those that are compiled using optimizations can see version dependence. As to the new features of GCC 4.0, as dwildesnl says: "it's a matter of having code that _uses_ them. :D"