The Fourth is soundless: unutterable, a quieting down of all the differentiated manifestations, blissful-peaceful, nondual. Thus OM is Ātman, verily.
Zimmer’s Mandukya-derived analysis establishes the four-part structure of OM—A as waking, U as dream, M as deep sleep, and the Silence as Turīya—as the complete symbolic map of Ātman-Brahman.
, Philosophies of India, 1951thesis