Here's the second verse of Mandukya Upanishad.
I have looked at a number of translations and I'm still unclear as to what verse two is saying.
What is it that subsists in the four states?
Is it Atman, is it Brahman, or is it Atman/Brahman?
See the various translations of this verse two here:
Röer For this all (represented by “Om”) is Brahma; this soul is Brahma. This soul has four conditions.
Aurobindo All this Universe is the Eternal Brahman, this Self is the Eternal, and the Self is fourfold.
Hume For truly, everything here is Brahma; this self (ātman) is Brahma. This same self has four fourths.
Purohit Swami and Yeats There is nothing that is not Spirit. The personal self is the impersonal Spirit. It has four conditions.
Swami Nikhilananda All this is, indeed, Brahman. This Ātman is Brahman. This same Ātman has four quarters (pādas).
Radhakrishnan All this is, verily, Brahman. This self is Brahman. This same self has four quarters.
Swami Gambhirananda All this is surely Brahman. This Self is Brahman. The Self, such as It is, is possessed of four quarters.
Mascaró Brahman is all and Atman is Brahman. Atman, the Self, has four conditions.
Easwaran Brahman is all, and the Self is Brahman. This Self has four states of consciousness.
Olivelle — for this brahman is the Whole. Brahman is this self (ātman); that [brahman] is this self (ātman) consisting of four quarters.