I did a bit of searching but couldn’t find much detail on QM-4. There were two Japanese industry standards for matrix systems. RM covered “surround oriented” matrix systems (e.g. QS) QM covered “front-oriented” matrix systems (e.g. QM4, Stereo-4 and QX).Somewhere in my digging in the past few months, I think I remember a quick blurb on the QM-4 format. I wanted to say it was yet another variant of Regular Matrix in Japan,
but of course I can't put my fingers on that right now. It's curious how QM-4 is it's own setting and not included as a sub-function under "matrix" (along with SQ and RM). It
might just be an ambient extractor along the lines of a Hafler circuit.
The fundamental difference between RM and QM was in the separation between the channels. RM has equal separation adjacent channels, QM has greater separation between front channels but lower separation between front and back channels.
(QX, a Denon invention, was slightly different in that it required a unique five speaker layout).
I don't think either QM-4 or QX were adopted outside of Japan.