More like 3.5 stars, rounded up.
In general, I tend not to like mysteries focused on tracking down a serial killer, but this story differed in that its focus is on the detectives and their interrelationships rather than on the criminal. (I nearly quit reading after the first page from the PoV of the killer, but that was the one exception, although hard to stomach.) The rest is told from the PoV of Maeve, whose partner and immediate superior, Josh Derwent is misogynistic, including toward Maeve, although, as time goes on, the author portrays a softer side to his character. He apparently is not alone among the male detectives although one of DI's is a no-nonsense woman who hates Josh. Maeve tolerates abusive language (nothing physical) from Josh that would have him reprimanded if not fired in this day and age, and I disliked him immensely (as the author intended.) Mauve recognizes that, despite his behavior, he is a very sharp investigator and, apparently, is willing to stick it out to learn what she can from working with him. Some of the exchanges between Mauve and Josh add elements of humor to an otherwise morbid story. She's feisty enough to hold her own, especially since she lives with a (former) colleague, Rob, who loves her and is the perfect opposite to Josh.
As the publisher warned, the three present murders are related to a 20-year-old cold case in which Josh was involved, so there is an element of doubt introduced as the other detectives treat him as a suspect in the present cases. There are others, former friends of Josh, who also materialize as persons of interest at the present time.
I realize that this is the fourth in the series, but the background was filled in enough early on that enabled me to enjoy this regardless. My wife read the book and also enjoyed it, and she is even less tolerant than I to stories involving gruesome events and perverted serial killers. That's a tribute to Jane Casey's writing. Even though I liked the story, there is a little too much suppression of facts known to Mauve and a touch of deus ex machina toward the end. I'd be willing to give another highly rated story in this series a try.