Waking, dreaming and deep sleep equally belong to something beyond them all. This is what the Upanishad wants to introduce us to. The reality about yourself, let's call it the fourth. It's called the fourth only in relation to the three aspects with which we are familiar. Which is right here, right now, but we are missing it.
Swami Sarvapriyananda teaches one of the shortest and most profound upanishads accompanied by Gaudapada's Karika. This lecture covers Karika 1-5.