The 4QM is one of the most useful analog devices in music synthesis, where absolute precision is unnecessary. There it's called a ring modulator because it emulates the sound of a bell. It produces both the sum and the difference of the two sinewaves fed into it.

Edit: Just looked up "ring modulator" on Wikipedia. It says the name comes from the ring of diodes in the canonical circuit. I'd always heard the genesis was from the bell-like sound it made. And there are better ways to build analog multipliers than four diodes in a circle.