Any basic cpp capability m4 cannot do?

My analysis suggests cpp and m4 share some 10 basic (non-trivial) 
capabilities, and m4 has some 15 basic capabilities that cpp cannot do. 
Examples of "basic capabilities" are: comment, define a macro, include a 
file, execute a system command, increment a number, etc.

My question: Can anyone suggest some basic capability cpp can do that m4 
cannot? Especially in the context of pre-processing C code.

I'm trying to anticipate what might happen on the next step of a 
conversion process, to anticipate something special cpp maybe does I've 
overlooked.

Thanks,
Daniel