That fourth volume is a painful and awkward read.
I agree with @Sol Falling and the synopsis they gave.
The one thing I wonder is:
Could the author have been trying to tell the story of a MC that is capable of a semi-romantic relationship while not realizing she's ace?
For three volumes Sahoko has been pursuing Aoi, and fairly successfully so, and is even delighted when she finds out it is mutual. Sahoko is attracted to her, and even enjoys some light physical intimacy with her.
It's only when Anna tries to force herself onto Sahoko that she becomes, for lack of a better word, "triggered" by physical intimacy.
That line "It's not romantic", I wonder if we are taking it too literally. What Aoi might be saying is that Sahoko can't have a physical relationship with her (in addition to a romantic one).
Anyway, just a thought.
The art by Fly is just wonderful, so too bad about not sticking the ending.
