Think I would give this 3.5 given the choice.
I really enjoyed the first 2 thirds of the book and it was a very solid 4 at that point.
The world building in particular, the 2 cities really came to life for me. I could really feel the tension between them.
The characters were well crafted and believable. The dynamics between them interesting.
The central mystery itself was absorbing.
The final book just doesn't stick any sort of landing for me. It feels incredibly rushed, almost like the author just had to get the book behind him and move on to another project.
Still gets 4 stars for me the ending is very weak but the journey towards it was engrossing and the world will stick with me for a while I think.