What's with these trilogies that all of a sudden sprout fourth books? I'd like to believe that it's all out of fondness for Douglas Adams' five-book "increasingly inaccurately named Hitchhiker's Guide to the Galaxy trilogy," but I doubt it.
Not to toot our own horn, but Terie Garrison's DragonSpawn Cycle was always a four-book cycle. We're happy to welcome Christopher to the club, though. Plenty of room.