The trademark of the HP4 is simplicity. There are four headphone outs in addition to the left and right line inputs and level-controlled left and right line outputs. The rear-panel Monitor outputs can be muted while the headphone outputs are still in use, the stereo signals can be combined to mono, and the separate headphone volumes may be changed. The HP4 thus has all the characteristics required to build a flexible headphone system while yet being easy to set up and use.
