This book is kind of an interlude between The Twin Trials and the next adventure. There isn't much action -- just one climactic scene -- and a lot of magic development. And the magic development is more tedious and harder to follow than the prior books.
Overall, not as good as books 1-3, but the story progression and world building bode well for what book 5 should bring.
I like the magic system in general, but there are times when the author is trying too hard. I read this book (not audiobook or anything), and had to re-read parts when Malachi was talking about his mana garden. And, even with rereading, couldn't really understand what was going on. Just too much theory.
There are a couple writing style annoyances that don't generally bother me too much, but in a slower-paced book show up more. One is that the author just LOVES using ellipses. Things like: "It was...strange." He's using the ... to get the reader to pause the cadence, but he does this trick all. The. Time. Including in scenarios where it really adds zero. A little editing would help. The other stylistic thing in this book -- and this one was NOT in the previous 3 nearly as much -- is the author using opposites to make something magical and mysterious. It was somehow both scalding hot and freezing cold somehow, both a void of nothingness and overflowing, all colors and none. Those kinds of things. Cool when used, once, or when used consistently around a specific thing, but this was maybe a dozen times throughout the book. Some light editing would really help fix these issues.