What's the difference betwen GCC 4.0 and 3.4?

[golfer]

I've already compiled a few programs and kernel modules.  When I
installed 'build-essential' it brought in gcc-4, so I thought that was
what was used for breezy, but when I tried to compile kernel-modules,
it balked and needed gcc-3.4.  I'm not sure now what to think.  It
seems my programs were compiled with 4.0 (since that's where
/usr/bin/gcc points), but the kernel is 3.4.  So, nothing has crashed
yet.

regards,