A dangling Belkar might be a cliffhanger - but I don't see why a chapter break is appropriate here. I don't really know what events will come next, but a natural point would imo be the arrival at Tinkertown (which I think will take some time).
Have you fought of potential round breakups for Book 5? I have done so myself, and I have a hard time to break them in small chunks. I would probably do something like this:
- (673-698) Desert shenanigans
- (699-709) Team Evil
- (710-774) Empire of Blood
- (775-822) Arena
- (823-833) Team Evil
- (834-898) Girard's pyramid
- (899-936) Advanced Escaping
- (937-946) Epilogue
ups, seems that I hit post reply instead of preview :/
Anyway I wanted to add some thoughts to it: I think the Team Evil/Epilogue splits are pretty easy. But I have a real hard time to split the other parts. I really think they are too long (though The Battle of Azure City was pretty long, too), but I just can't find a spot I'm really satisfied with.