Why would Mokou be a Stage 4 midboss? I mean, it does make some sense, seeing as Stage 3 takes place in the forest, but maybe Stage 4 is somewhere completely different. Besides, Mokou is human- a youkai rebellion wouldn't affect her at all.
Oh well. If Stage 4 is still in the bamboo forest I would say that the midboss would be a certain mischevious youkai animal who leads a whole species of youkai Tewi
Also, DDC and PCB are the only two games where Cirno is a midboss and Sakuya, Marisa, and Reimu are playable. I'm calling another Phantasm boss for this game