So what is the Fourth?
Mandukya upanishad tells us that: “The Fourth, the Self, is OM, the indivisible syllable. This syllable is unutterable, and beyond mind. In it the manifold universe disappears. It is the supreme good -One without a second. Whosoever knows OM, the Self, becomes the Self.”
And Taittiriya upanishad continues: “Man, in his ignorance, identifies himself with the material sheaths that encompass his true Self. Transcending these, he becomes one with Brahman, who is pure bliss.”