For the people who are looking for an explanation of what happens in Vol 4, the basic plot is Yomotsuka recruits Itsumi to help bring an absentee yakuza princess back to class, shenanigans ensue.
As opposed to the rest of the series, which has been largely one shots beginning with Itsumi trying to drug someone and getting her comeuppance, the entirety of Vol 4 is one complete story and Itsumi doesn't get to use her delicious dogbane tea until the very end. The "problem" is that it's not a problem centered around girls kissing girls, it's more of one big action set piece
It's kind of like the "movie" part of "seven seasons and a movie," it serves as a kind of greatest hits by bringing back a bunch of recurring characters from earlier in the series working together towards a shared goal. The fun is less about getting more of what you usually get and more about celebrating your favourite characters. It's a good once-off kind of thing, the previews for Vol. 5 make it look like things are going straight back to the way they've been.