Turiya is Atman (Soul) the innermost Self itself. On the standpoint of the Atman, the innermost Self, There is no fourth state. On the standpoint of the Atman, the three states are mere an illusion. The Atman, which is present in the form of the consciousness alone, is real and eternal.
To realize this truth the seeker has to realize first “What is the truth? and “What is the untruth?” between the dual (waking/dream) and nondual (deep sleep) experiences. All accumulated bookish, scriptural knowledge is not the wisdom.
Holding some Gurus teaching as wisdom without verification blocks the realization of the Advaitic truth, which is beyond the form, time and space.
That is why Sage Sri Sankara says: ~ What is accepted without proper inquiry will not lead to the final Goal. (Commentary on Veanta Sutra).
Sage Sri, Sankara:~ Vc.63 :~ Without knowing and examining the external world, one can’t know the Truth, as the idea that the external world exists, won't go. It can go only by an inquiry into the nature of the external world.