4 Hz marks the upper edge of the delta brainwave band — the boundary between the deepest unconscious sleep and the lighter, more dream-prone states associated with theta activity. It is still firmly in deep-sleep territory, but carries a quality of transition — the brain beginning to stir toward higher-frequency states.

At 4 Hz, the brain is still producing the slow, high-amplitude waves that characterise restorative sleep, but the first hints of theta activity may begin to appear. This makes it useful for people who want to encourage deep sleep without dropping into the absolute depths of 1-2 Hz — a gentler approach that may feel more natural for some listeners.

An interesting footnote: 4 Hz is also the most common frequency used in mechanical wristwatch movements — a curious convergence between the rhythms of precision engineering and the rhythms of the sleeping brain. Here it is delivered as a binaural beat: left ear at 148 Hz, right ear at 152 Hz.