This is the “odd man out” in the quad world. Extremely ‘fine’ grooves were etched onto the vinyl record to provide frequencies between 35,000 and 50,000 hertz. The CD-4 demodulator sensed these high frequencies then converted them to a range of around 100 to 15,000 hertz. These frequencies were then sent to the amplifier and on to the rear channels.
A matrix decoder can not sense the high frequencies on a CD-4 record, it can only synthesize the stereo output of a CD-4 QuadraDisc LP.