Castor has a problem -- several problems, but the long-running one is that his best friend is possessed by Asmodeus. He does not have a fix for this (sorry), but he can help a little, for example by cracking his friend out of a psych ward. (This doesn't mean relaxing the anti-Asmodeus precautions, but they can at least get the straightjacket off.)
A good deed, this; but also kidnapping and trespassing and probably indecent exposure (Castor's friend the succubus helps out, which must count as indecent something), and you don't want the cops mixed up in that. Unfortunately the cops come looking for Castor anyway. Something about his name found written in blood.
Book four is traditionally when a fun-and-guts urban fantasy series starts getting good. The author has had time to chew the scenery, settle in, and start thinking about long-term character arcs. This series was pretty darn good already, and book four is a major step up. See, it's about family: Castor's family, the family of his family, and all the little and large screwed-up families around him. They're all broken and trying to live anyway. Some mostly succeed, some mostly fail. They're all very human. (Even the one which includes a succubus.)
And this wraps around a lot of pain, a whole lot of narrative tension, several linked unpleasant revelations, and a dirty, dirty cliffhanger. I will be moving on to book five very soon.