1. A GNU implementation of the traditional UNIX macro processor. M4 is useful for writing text files which can be logically parsed, and is used by many programs as part of their build process. M4 has built-in functions for including files, running shell commands, doing arithmetic, etc. The autoconf program needs m4 for generating configure scripts, but not for running configure scripts. Install m4 if you need a macro processor. From Mandrake 9.0 RPM

2. a macro processing language 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). `m4' also has builtin functions for including files, running shell commands, doing arithmetic, etc. Autoconf needs GNU `m4' for generating `configure' scripts, but not for running them. From Debian 3.0r0 APT