I'm surprised this got a volume 4, but think that it needs to make a few changes if it wants to keep going. Sure it's mostly story-based with the erotic art as a side element, but I feel like there's too many scenes with the main girl who's mostly "just okay" in terms of attractiveness, and not enough scenes of other girls for different types and fetishes. It doesn't even have to be the main guy doing stuff with other girls, just somebody doing something so that we have more variety than the main couple / main girl.
The main girl's best friend never got a full scene, and no one's done anything with the back hole yet, so it feels like the h-scenes at least are getting stale.