The QSD-4 is a single-band Vario-Matrix decoder for QS. It's a pro model and has better quality circuitry than the consumer models. The QSD-1 is a better decoder though due to its ability to decode in 3 bands, unlike any other Vario-Matrix decoder, including the vaunted QSD-1000.
The QSD-4 was used in the BBC's large-scale tests of the various matrix quad systems (For SQ, the awful CBS Labs Model 2400 Full-Logic decoder was used) because the BBC wanted only the most advanced professional gear, which at the time, the QSD-4 was. For decoding QS, while it was better than SQ's logic decoder, the BBC still didn't like it too much but modified it to decode Matrix H, then the BBC used the Vario-Matrix chips to create a truly custom and more advanced Matrix H decoder. The QSD-4's big failing in the BBC tests was the side-to-side shifting of instruments and the soundstage due to QS requiring side-to-side enhancement.