I know FreeBSD 7 is moving to GCC4 for the system compiler, iirc 7.0-Release comes with 4.2.x. I have GCC 4.3 installed on this laptop but my primary reason for it is that I wanted to try GCJ, the GNU Compiler for Java :-).
I can't say the same about Linux distros but I'd expect FreeBSD to always make sure the systems compiler won't have problems with a build world/kernel cycle.
I've only used GCCs 3.x.x C and C++ compilers but I'm not partial, all I expect from a compiler is sufficient standards compliance to the current standards. GCC 3.4.6 might not be perfect but hey, it beats Microsoft's C Compiler hands down last I tried it :-).
I used to collect some types of software, text editors, terminal emulators, shells, window managers, and sometimes even office suites but I never took to collecting C Compilers. Largely because I don't want to sit and wait to compile one more often then needed haha.