The C4 compiler is certainly fascinating and worth studying but don't believe that swieros's C compiler is based on that judging by their code [1][2]. There are similarities of course but well they are compiling the same language after all using some similar standard techniques like recursive descent parser [3].

Anyways, C4 is also very cool and focused on minimalism. Swieros's compiler seems much more extensive and even includes #define as the other comment mentioned and va_args.

[1] https://github.com/rswier/swieros/blob/master/root/bin/c.c

[2] https://github.com/rswier/c4/blob/master/c4.c

[3] https://en.wikipedia.org/wiki/Recursive_descent_parser