...and you will be wondering if Book Four actually worked, because you're continuing those story threads in Book Five, and suddenly, you panic - because you are, for the moment, CONVINCED Book Four is awful. And no one knows it because it isn't out yet. And worse, you can't fix the awful, because it's done - and you have to make the awful somehow good, but you have to do it in Book Five.
And there's only one way to make Book Four good again - that's by finishing Book Five, and moving ahead with Book Six (at which point Book Four is suddenly transformed into a work of genius, and Book Five is suddenly crapola.)
It's a bit maddening, yeah - but it's how it works. For me, anyhow.