The parts of a Buddha are also divided into four parts, sku bzhi. In this case the two form parts are the same, but the non-form parts are two. We went through ye shes chos sku before. The second part is ngo bo nyid kyi sku - svabhavikakaya.
ngo bo is essence, svabhava in Sanskrit; one of those words good to recognize, as you could see it used in various other terms in the scriptures.
nyid means itself, the very one, exactly.
The term ngo bo nyid is the very essence, the essential nature of something.
kyi is a genitive particle, binding the right side to the left, sku is body. Thus the rough translation the body of essence, or the Essence Body as this has usually been translated in English.
So what is this essence body? Depending on the tradition you get different answers. One is that this is the emptiness of the three other bodies. Thus, the three bodies are united in the form of them lacking self-existence, and appearing due to dependent origination. Sometimes this is described as the essential nature of being itself, something that is totally pure in nature. It is the ultimate buddha part, the wisdom aspect is dharmakaya, and the form is rupakaya, where pure beings perceive sambhogakaya, and impure beings perceive nirmanakaya.
But wait, there's also a fifth part, that's next!