A macro processor for Unix and GCOS which is more flexible than cpp. m4 copies its input to the output, expanding macros which can be either built-in or user-defined. m4 has built-in functions for including files, running Unix commands, doing integer arithmetic, manipulating text in various ways and recursing. m4 can be used either as a front-end to a compiler or as a stand-alone tool.
sendmail's configuration file (/etc/sendmail.cf) is writen in m4 macros.
There is a GNU m4 v1.1 by Francois Pinard
["The M4 Macro Processor", Kernighan & Ritchie, Jul 1977].