First off: apologies, chapter 20 is a bit on the short side. It's mostly the "preparation before the big adventure" chapter, but you can expect chapters 21 and 22 to be much larger.
Secondly: good news! Book 5 has been sent to Hosted Games for publishing! All the editing, the intro work, and the save importing stuff is done. There's still a long (and ever growing process) to go through, though, and many of you know how lengthy the queue for releases is!
The estimated release of September HG/CoG put down several weeks ago is exactly that: an estimate. When they made it, they weren't aware of how close I was to finishing, so I think we can expect it sometime sooner than that.
Or maybe that's just my hopium talking, lol!