Mahavakyas of Advaita or Vedanta

The Mahavakyas -- or great sayings or principal statements -- from Upanishads are said to contain the essence of the teachings of Upanishads. It is said that meditating on the Mahavakyas would lead someone towards realization.

Pranava (Aum): Pranava is said to be the fundamental (true) Mahavakya of the Vedas as established in Chapter Twelve. (details?)

For a particular Vedantic system, some of the Mahavakyas are considered more important than the others. This is inspite of the fact that all schools in Vedanta accept Upanishads and all the Mahavakyas are from the Upanishads.

According to Arvind Sharma, Sri Adi Sankara stated that four Mahavakyas are fundamental for Advaitic learning. A web search also shows that Sri Adi Sankara set up the four schools, one for each Mahavakya. (Yet to find out which school stands for which Mahavakya.) They are the following:

1. Sarvam Khalvidam Brahma: meaning All this verily Brahman. This is from Chandogya Upanisad, 3.14.1.
   
   
2. Aham Brahmasmi: I am Brahman. This is from Brhad-aranyaka Upanisad, 1.4.10.
   
   
3. Ayam Atma Brahma The Self is Brahman or This Atman is Brahman. This is from ?
   
   
4. Tat tvam asi: That thou art. This Mahavakya is from Chandogya Upanisad, 6.8.7 (Tat tvam asi svetaketo, "O Svetaketo, you are that")
   
   

Additional Mahavakas from Vedanta. Some of them are:

Prajnanam Brahma: Brahman is Supreme Knowledge prajnanam brahma, This is from Aitareya Upanisad, 1.5.3.

So'ham: He am I. This is from ??