GNU M4 is an implementation of the traditional Unix macro processor. It is
mostly SVR4 compatible although it has some extensions (for example, handling
more than 9 positional parameters to macros). GNU M4 also has built-in functions
for including files, running shell commands, doing arithmetic, etc.

GNU M4 is a macro processor in the sense that it copies its input to the output
expanding macros as it goes. Macros are either builtin or user-defined and can
take any number of arguments. Besides just doing macro expansion, m4 has builtin
functions for including named files, running UNIX commands, doing integer
arithmetic, manipulating text in various ways, recursion etc... m4 can be used
either as a front-end to a compiler or as a macro processor in its own right.

One of the biggest users of GNU M4 is the GNU Autoconf project.