The local gcc is a modified version of gcc 3.3.5, they simply don't bump version counters.. it's inappropriate because it's not their project.
There is a man page documenting the changes to the compiler, most apparent are the stack protector changes..
It's a good read,
gcc-local(1), I'm not sure, but I think the "gcc 4.2" port can coexist with the system compiler, one is in /usr and the other is in /usr/local.