The moral of the story is that if a proposed new book is closely related to the preceding ones, it will probably work better as a fourth book in the old series than as the starter of a new series.
This is what I'm thinking. I'm leaning towards a Book 4.
Would you rather reach new readers or sell more books to readers of the series now and in the future? Do you advertise the book 1 of this series a lot? How is your sellthrough?
I'd love new readers while keeping the old.
But I see your marketing point here.
This will be book four in my highest selling witch series. I advertise Book I more than anything else. sell-through is good but no where near sales of the 1st book. The way I see it, if I made this new series separate, I'd have to advertise Book 4 separate. This could work but it also could cost more money and less revenue.
I'm not sure yet, as I'm still in drafting phases and I've only drafted book I, whether these new books can be understood without the preceding series. I'm pretty sure I can make it clear enough and get book one, or "four", to be understandable without reading the preceding books. I'd like to achieve that. If I do, and I write three as planned, I should be able to re-title it a "new series" later if I so choose. I can also create a new boxed set.
The series was written two years ago. There will be a problem with connecting past readers^^, but fans will like it and book 4 will be definitely connected by vibe and with the same characters (Incidentally, in the past, I'd take a break with a book series. I'd write a book in one series then switch to another. This one I'm going to work straight through for the purpose of close launches).
If book 4 fails to sell well, I
could move it away from the original series and work on marketing it separate later. But optimally, I lay my cards down right at the start with its launch.