Welcome Paul! Each of the settings you mentioned are different ways for the receiver to decode four channels of sound from a stereo or encoded (into two channels) quadraphonic source ... in the quad days that would have usually meant LPs. SQ LPs were released mainly by CBS/Columbia/Epic labels (affiliated with Sony), and QS ... labeled R-MTX (for Regular Matrix) was from Sansui, but had far fewer available LP releases ... although most believe that it was the superior system.
The 2-4 setting is sort of like the R-MTX setting and was there to decode four channels from stereo LPs. Try them all on some of your stereo CDs and see what you think. My guess is that the SQ setting will not do much for you that way, but you should get a fairly decent surround sound effect if you play back your CDs through either of the other settings. Cheers, Mike.