The End of a Trilogy

Somehow I forgot to post this one so here it is! So after contemplating which book I would choose from the large pile of books I brought home for my summer reading I ended up going to the library and I came home with The Prophet of Yonwood by Jeanne DuPrau, the third book of Ember.  I read the first two Ember books over the spring after Allie recommended them to me.  Mrs. Perkins had recommended them to me also. I really loved The Book of Ember and I liked The People of Sparks, but this book, which supposed to explain the existence of Ember, did not live up to my expectations.

The Prophet of Yonwood tells a story of a young Nickie Randolph's journey to Yonwood a small town did not really get to the point for me until the final chapter when the main character, Nickie, explains that her father was working on the underground city as part of his super secret work, and Nickie herself, when she was sixty years old, was one of the first residents of Ember and wrote the journal that was found when the kids escaped out the river from Ember.  It was meant to be a "letter to the future" but there wasn't much that was written.  Nickie had much more to tell than she did.
 Now I've found out that there is a fourth book The Diamond of Darkhold.  I guess it's not a trilogy after all.  Would it be a "quadrilogy"?