except for (really) really specific code there's no changes between 3.x and 4.x ...
Anyway:
there's no gcc 4 package for now (as far as i can tell). Mostly because it can break a lot of things for people not taking care of version (kernel modules, glibc, ...)
We're waiting for the slackware's new toolchain (new gcc, new glibc, new ...) to use gcc 4.x package.
If you recompile it take care to prefix/suffix the 4.x version not to break "system's compilation" (i.e. nvidia driver, kernel modules, kqemu, ...)
(and specify CXX=yourgcc in your makefile, of course
)