Source : Free On-Line Dictionary of Computing

m4
     
        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
        (ftp://gnu.org/pub/gnu/m4-1.0.tar.Z)} by Francois
        Pinard  and a {public domain} version
        by Ozan Yigit  and Richard A. O'Keefe
         (FTP from any {386BSD}, {NetBSD} or
        {FreeBSD} archive).  A {Macintosh} version is {here
        (ftp://nic.switch.ch/pub/software/mac/src/mpw-c/)}.
     
        See also {m3}, {m5}.
     
        ["The M4 Macro Processor",
        Kernighan & Ritchie, Jul 1977].