The fourth is without an element, with which there can be no dealing, the cessation of development, benign, without a second. Thus Om is the Self (Ātman) indeed. He who knows this, with his self enters the Self—yea, he who knows this!

In plain words: The fourth state has no sound part of its own; it cannot be dealt with, it is where all change stops, it is good and has no second. Om is truly the Self, and whoever knows this merges their own self into the highest Self.