The Mandukya Upanishad describes Turiya as following:-
“Turiya is not which is conscious of the inner (subjective world), nor that which is conscious of (objective) world, nor that which is conscious of both, not that which is mass of consciousness. It is unperceived (by any sense organs), incomprehensible (to the mind), unrelated (to any object), uninferrable , unthinkable, and indescribable. It is essentially of the nature of consciousness , constituting the self alone, and is the negation of all phenomena; it is the peace, bliss and One without a second. It is known as Turiya. It is Atman, and it is to be realised.
Shankarcharya wrote the following hymn in praise of Turiya:-
“ I bow to Brahman, which experiences (during the waking state) gross objects by covering the universe with the tendril like rays of its consciousness , enfolding all movable and immovable entities; which, further, experiences during the dream state the objects produces by the mind due to desire; and which again in deep sleep, absorbs the various particulars and enjoys bliss, while making us also experience, though maya, the same bliss- I bow to the Supreme, immortal, and the birthless Brahman, designated in terms of maya as Turiya, the Fourth.
May the Turiya, which , as the World Soul, experiences in the waking state gross objects, good and evil; which, again experiences in the dream state other and subtle objects produced by it own mind and illumined by its own light; and which, lastly, in dreamless sleep withdraws all objects and remains devoid of distinctions – may that attributionless Turiya protect us!!”