Here's some news.
I have over a half million words of book five and six and seven and etc. Chapters, scenes, interstitials. More.
Some of it is out of order. Some scenes are outlined, not written.
I am now working on a spreadsheet to chart (outline) all the separate pieces of every chapter all the way to the end of the series. I was hoping to avoid the extra work, that was a mistake. This is the only way to figure out what has to go where. (There's a lot of flash-forwarding in book five and that has created problems with book six and ... that's forcing a decision about.... )
Anyway, the spreadsheet is now over a thousand separate records, because there is just that much that has been written.
I expect to finish the spreadsheet within a week. I expect that will speed up the completion of the project.
The spreadsheet alone is turning into an interesting document.