The 7th edition manual[1] says m4 was based on m3, which was based on the Software Tools macroprocessor. The source for that is at https://9p.io/cm/cs/who/bwk/

The 7th edition manual[1] says m4 was based on m3, which was based on the Software Tools macroprocessor. The source for that is at https://9p.io/cm/cs/who/bwk/toolsbook/index.html https://9p.io/cm/cs/who/bwk/toolsbook/index.html 1. e.g. https://wolfram.schneider.org/bsd/7thEdManVol2/m4/m4.pdf https://wolfram.schneider.org/bsd/7thEdManVol2/m4/m4.pdf