Every Upanishad must have a maha-vakya महावाक्य, not just four Upanishads. For the sake of समन्वय samanvaya, showing that all four Vedas have only one तात्पर्य tattparya, vision, four maha-vakyas are quoted, one from each Veda:
- तत् त्वम् असि, Tat Tvam Asi -> That Thou Art.
from Chandogya Upnishad, Samaveda.
- अहम् ब्रह्मास्मि, Aham Brahmasmi -> I am Brahman.
from Brahadarnayaka Upnishad, Yajurveda
- प्रज्ञानम् ब्रह्म, Prajananam Brahma -> Consciousness is Brahman.
from Aitareya Upnishad, Rigveda
- अयम् आत्मा ब्रह्म, Ayam Atmā Brahma -> This self is Brahman.
from Mandukya Upnishad, Atharvaveda.
In fact, every Upanishad has maha-vakya. Without a maha-vakya there is no Upanishad, there is no Gita, and there is no शास्त्र sastra either. Any sastra reveals what is to be revealed, and therefore, maha-vakyas are seen in all the Upanishads.
In maya-vakyas there are no differences. It is not proper to create differences among them, like some people do. Some claim that, tat tvam asi is an upadesa-vakya, a sentence giving the teaching; aham brahmasmi is an anubhavakya, a sentence revealing the experience of oneness, and so on. The whole Upanishad is meant for upadesa, revealing an equation between जिव Jiva and ईश्वर Isvara.