WARNING: If you're reading this but haven't read books 1 to 3, stop wasting your time here and go read Book 1. Immediately. Seriously.
Now, on to the important stuff. I've had my issues with this series so far - finding the writing style a little too formal for my liking, hugely convoluted and oftentimes fragmented multiple storylines, loads (too many for my tired brain at times!) of characters to keep track of ... but despite all of these niggles there's been something that's kept me faithfully coming back for more. Book 4 is solid evidence that this series is going somewhere really, truly exciting, and is itself a series of revelations and shockers that had me turning pages long after bedtime.
As I was finishing this book, my brain made a random connection to Arthur C Clarke's 'Rama' series (of which I was a huge fan although similarly in two minds about stylistically). The final chapter of Book 4, title and all, just felt like one of the 'false' endings in the Rama series, where the immediate storylines were all tied up, but you knew something bigger and even more awesome was on the way. In that respect, I'm fully expecting Book 5 to be a jaw-dropper ...
I'm also beginning to understand the subplot much better, and the more I understand, the more its genius is revealed. At first I thought this series was just loosely based on The Divine Comedy (as was the author's intention, I would presume), but it's much more intricate and creative than that, and once realisation hit it took the narrative into completely new territory and made it ultimately even more compelling.
I do still struggle with the formality of the writing which, particularly when switching to the first person account, can feel a little forced, and I'm also still struggling to keep up with all the characters and individual storylines, but these pale into insignificance when the overarching narrative structures start to assert themselves, and things begin to converge. I also had a bit of an issue with the method by which the plot refreshers were delivered in this one - it had been a while since I'd finished Book 3, and I was hoping for a 'Previously, in AngelFall ...' lead-in chapter, but instead the plot refreshers were delivered along the way in an 'As you may recall ...' fashion, and often via dialogue (which I found particularly bizarre). I'd much prefer a refresher preface. He hinted.
So, in conclusion, if you didn't heed my warning at the start and are still considering whether or not to jump in to this, in all seriousness, go and start book 1 now. Just make sure you have books 2, 3 and 4 lined up once you're done. This one was the best yet, and I have high expectations for Book 5, due for release in June 2013.