Actually, in CD-4, the front channels and the rear channels are the same quality. During the encoding process, the front channels are mixed with the rear channels in phase and recorded as the main audio on the audible part of the bandwidth. In this way, if you played it on a stereo system, you would experience it as a stereo record, and all the musical elements would be there with left/right separation maintained by vector modulation same as stereo. The subcarriers, there were two, one for each of the two channels, were separated and sent to two channels of demodulation. After demodulation there was ANRS noise reduction, a system similar to Dolby B. These subcarriers are the difference signal generated by mixing the front and rear channels out of phase, this is the separation information that is used to reconstitute the original four channels. To do that, the separation signal is mixed in phase with a sample of the main signal, and the rear signal is canceled out because it was out of phase when mixed in the modulator. This leaves only the front channel which is sent to the output. To get the rear channel, the same two signals are mixed, but the separation signal is mixed out of phase, so that the front channel audio is cancelled out leaving the rear channel signal which is sent to the output. Never are the left and right sides intermingled. So the result is that all four channels are the same quality.And the quality can be pretty good with the right setup. There is delay compensation in the modulator designed to correct for the natural delay in the demodulator. This was done to make the demodulators less expensive because they would not need to have their own compensation. CD-4's best performance was finally realized after the quad era, with the development of the microline stylus, or linear contact, a Shibata variant that performed better at tracking the finer modulations of the subcarrier frequencies. The CD-4 system is similar to FM stereo, except that FM stereo does not use noise reduction for the subcarrier.
With SQ and QS and other matrix systems, the left and right channels are intermingled, but they do start out as four independent channels. The encoders mixed the four channels into the two channels of stereo at different phase angles, which the decoder would try to extract. The problem was that the decoders could extract some separation, but not enough. It wasn't until the Tate system was developed that serious separation performance was realized for SQ, and Variomatrix for QS, but by that time, quad was commercially dying. It also didn't help that every cheapo stereo manufacturer had a fake quad system which consisted of a stereo with a Hafler circuit and a "quad" button, that made it sound different, but wasn't true quad.Many people thought they had heard quad, but never did.They were disappointed, of course.
The Quadfather