WikiGentoo has a frontend tool for emerging cross-compiling toolchains. Simply put, this allows you to emerge a cross-compiler toolchain with one command.
First, emerge crossdev:
$ emerge crossdev
Then tell it to emerge a SH4 toolchain:
$ crossdev -t sh4
Once this has finished, you should have a full C/C++ cross-compiler for the SH4 cpu. If you just want a cross-compiler to build a kernel, you can save yourself sometime:
$ crossdev -t sh4 -s1
This will install just a C compiler (no libc) so you can build kernels (and not much else).
$ sh4-unknown-linux-gnu-gcc --help
By default, crossdev will pick a glibc based toolchain. If you wish to utilize uClibc, you need to provide some more info:
$ crossdev -t sh4-pc-linux-uclibc
Then you have access to it with a simple:
$ sh4-pc-linux-uclibc-gcc --help
For more info, just run:
$ crossdev --help