I finished book 3 this morning and immediately wanted to read this one.
It was a good conclusion to the series, combining elements of all three previous books. Deception, time shenanigans and nifty historical details with a fun twist. Each character has their moments, even though they are split up for a lot of the time. After 3 books, they also feel more fleshed out and I've grown to care about them.
I did find the resolution of the overarching plot regarding the 445 mystery a little easily resolved. For a series that did not sugarcoat grim historical realities, drops a few swear words here and there and is not afraid to describe death, I expected something a little more dramatic than a short conversation.
If this series ever continues, I will be the first to dive into it.