"Quad band" simply means four independent frequency filters working together, each responsible for a different slice of the audible spectrum.
In this tool the four bands sit at 60Hz (low shelf), 500Hz (low-mid peaking), 2.5kHz (high-mid peaking), and 8kHz (high shelf). A shelf filter adjusts everything above or below its frequency; a peaking filter targets a narrower range around its center point.
In audio engineering, a "band" refers to a specific range of frequencies rather than one exact pitch — the low band, for instance, covers everything roughly from 20Hz up through 250Hz, not just 60Hz itself. That's why a small number of well-placed bands can still cover the entire audible range.