I'm not sure I feel comfortable calling Female Prisoner #701 Scorpion: Beast Stable the best of the series, but it certainly is the boldest and probably, as such, the most fun. Director Shunya Ito seems to have blossomed into full confidence in the wild over-the-top directing style that makes these feel more like horror movies than action movies. The scene in which two abortions are shown in contrast to one another is both fantastic and harrowing.
Female Prisoner #701 Scorpion: Grudge Song replaces director Ito with Yasuharu Hasebe who seems to try in some places to mimic the style, but mostly falls flat with it. This is also the worst script in the bunch, pitting her against a obsessed policeman who, as far as I could tell, has no personal grudge against Scorpion.
It also features a particularly crass rape scene that advances the plot in kind of a way, but plays mean-spirited and overly complicated in context. I know, this is a genre that relies on sexual violence as part of its draw, but one of the thing that I liked about the first three in the series is how they draw that to feel organic to the story and ultimately drive the characters forward. This was just ugly, boring and unredeemed.
Meiko Kaji is a fantastic driving force in all four. It's too bad that after the stunning display of the third movie that she's kind of hung out to dry in the final film.