The M4U 4 are in-ear headphones, developed by PSB Speakers, that is targeted towards the discerning audiophile. These headphones offer a fusion of proprietary Roomfeel technology and hybrid dual-drivers. When the headphones' audio signal reaches the in-ear monitor, lower frequencies are sent to a dynamic driver. This hybrid system has allowed PSB Speakers' designers to return the acoustic signature of a typical listening room.
The highlight of the M4U 4s is its ability to deliver extremely intricate and detailed high frequency sounds. This is thanks to the presence of individually calibrated balance armature drivers. Lower frequencies are handled by a tuned moving coil driver, which works in tandem with a trio of separate chambers to smoothen out the response and reduce resonance, delivering what PSB Speakers escribes as a "deep, deep bass response that is punchy and crisp without sounding fat or boomy."