M4 (Macro Processor Library)
The M4 Developer file format
M4 files are saved in a plain text format which allows them to be opened with any text editing program. They were designed for a Unix macro processor using the m4 language. M4 is part of many Unix-based operating systems, including Linux. The main purpose of M4 files is to set up configuration files.
Technical details of M4 files
The content of an M4 file is made up of text and macros. When processed by M4, the macros inside the file are extended. M4 files are an easy way for developers and programmers to re-use certain text. The macro code inside the M4 file follows a very free syntax.