There is also a larger project by Robert Swierczek here: swieros. it’s a complete suite of a software emulated “CPU”, simplistic OS and runtime environment in Plan9 (or rather Inferno) style, all in one pedagogical package. It also contains a much more featureful version of this C compiler.
Also, there are several interesting forks that add other functionality to c4:
Can any of these compile their own source?