"In the Hunt" has a warning of sorts at the very beginning, pointing out that the essays were written before season four aired, but I didn't find the essays to be any less enlightening or fascinating.
I do have to say my curiosity made me skip to read the essays on the Impala first, before going back to read from the beginning to end. I thought all of the essays were informative and some of essays were just plain hilarious too. I would find myself either mentally nodding along in agreement with the essays or finding even more connections and interesting points that I hadn't considered or noticed before. I also found myself having an even greater appreciation for all of the efforts of the writers' in tying in all the mythology/folklore into a show that's not just a monster-of-the-week show.
I think anyone who enjoys digging into the nuances of the show would like this book. Hopefully, after a few more seasons and/or the show ends, they'll be another book that explores everything that has happened since this book was published.