I'm inclined to say that Yotsuyu came back completely after the meeting with her forster parents, but her staying with Gosetsu had an effect on her, in ENG she says if only she didn't remember and wonders why Gosetsy doesn't hate her, she doesn't speak his name so I'm just assuming it's him, because on their relationship during 4.0. I think in a way that she reminds of herself and how she was stepped on by others and while she choose vengeance and hate, he choose compassion and kindness. This is why he appears in the adds phase or so I believe it is from what I can gather. It's possible we won't never be able to know for sure 100%, because there's a bit of out of character for her. I suppose they wanted to play a bit with it, the facxt that beside the initial scene we don't have a Yotsuyu and Gosetsu scene again makes things hazy in my opinion. .