Book 4 isn't up to par because the concept wasn't finished from the very beginning. They made the error to tease for Jinora content in the chibi comics and in some early scenes (like the sperm drink) without really having plans to make her a real side character. When people started to ask for her she was implemented terribly, without pacing and not enjoyable at all. But the main cause for all of this is the failed card game and the things that followed. Book 4 is full of such failed experiments and it shows. The quality of scenes is still improving/staying the same though, it's incoherent story and character development that's unpleasant (for me). Mity knows though and I hope he takes his time to change that with the love route.