The Fourth is in reserve, and will only reveal herself if need be.
Consider the following.
The Devil is Yuri "as a Mother", preoccupied with sex and childbirth. POP POP POP!
The Angel is Yuri "as a Woman", born from Yuri's addiction to Boysmell Darphemo.
Therefore, the split that has yet to reveal herself is Yuri "As a Guard Student", and is preoccupied with protecting Sou and earning Sou's approval. She suggested the Hug Compromise in chapter 243, and was actually carved off when Yuri first met Sou and realized he wasn't Like Other Boys.