Yeah, for whatever reason, Jefferson Mays wasn't available for Book 4, so they had Erik Davies do it instead. It's not like Davies isn't a fine narrator, but you get used to the voices, you know?
I think you'd do fine looking up the plot on-line somewhere. As far as I can tell Book 4's events don't really act as an anchor for any of the other books.
That said, it's not a TERRIBLE book. The writing is fine, the plot chugs along fine. It's just a bad book when compared to the others in the series, but there's no reason you couldn't go back and read it later.