The blurbs all over this book make it sound like the series will be continued, but the book itself reads as though it is the conclusion to this quartet, and I believe I've heard that the author themselves mentioned that they had no plans to continue this series, so I'm going to safely assume that this is the last book.
It's a good ending, all the loose ends that I can think of are tied up neatly, and there were a few plot twists that I really wasn't expecting. Throughout the whole book, I thought I knew what was going to happen, but this author surprised me yet again.
Certain aspects of the last few pages seemed a bit rushed, as there were some short winded explanations of what happened after the climax, but I guess it doesn't matter too much; it gets the point across.
All in all I thought this series was pretty good, I probably would have enjoyed the first two books a lot more if I had read them closer together instead of about 14 months apart, but hey, what can you do?
If you're intrigued by the synopsis of these books, I recommend taking a look at them.