Actually, the letter was written long before the book was actually published (before the Ellimist Chronicles came out, I think), and there was a 5th MegaMorphs planned at the time. However, KAA felt it was kind of stepping on the ending and didn't cover enough new thematic ground to justify a whole extra book, so it kind of melded into #54. She wrote about it in a follow-up letter defending the ending somewhere, forgot where it was.