My 4yo pretty consistently refers to herself as a boy. She's never said, "when I was a little girl"; it's always been "when I was a little boy" -- ever since she could talk. She sometimes says she's pretending to be a boy, other times she says she is a boy. She prefers boys' clothes, will never wear dresses, etc. Again, this has been the case since she was very small and could express her preferences. So if it's just a "phase", then it's quite a long one.
I think I read that gender awareness and identity begin to develop more fully around this age, but I suspect it's still too early to know for sure how she's going to identify. I was wondering, though, if anyone with very young trans kids could share their experiences as to when it became clear that the child was trans.