Not inwardly cognitive (antaḥ-prajña), not outwardly cognitive (bahiḥ-prajña), not both-wise cognitive (ubhayataḥ-prajña), not a cognition-mass (prajñāna-ghana), not cognitive (prajña), not non-cognitive (a-prajña), unseen (a-dṛṣṭa), with which there can be no dealing (a-vyavahārya), ungraspable (a-grāhya), having no distinctive mark (a-lakṣaṇa), non-thinkable (a-cintya), that cannot be designated (a-vyapadeśya), the essence of the assurance of which is the state of being one with the Self (ekātmya-pratyaya-sāra), the cessation of development (prapañcopaśama), tranquil (śānta), benign (śiva), without a second (a-dvaita)—[such] they think is the fourth. He is the Self (Ātman). He should be discerned.

In plain words: The fourth state is beyond waking, dreaming and deep sleep. It cannot be seen, grasped, thought about or described in words; it is pure peace, goodness and oneness with nothing separate from it. This is the true Self, and we should come to know it.