After the release of vol. 4, volume 4.5 was released that just contains primarily spin off stories - nothing in the book is essential to the main plot and I think that splitting it as it's own individual spin-off book would be better.
It would make things a lot easier when it came to managing chapter and volume count, like do I add them to the total or not? The actual series is at volume 5 so having 6 volumes on the list would be confusing, same with the chapter count. Either way it'd be nice to take it into consideration.
https://www.goodreads.com/book/show/200728066-secrets-of-the-silent-witch-vol-4-5--after-
Given it's sequential and named the same as considered part of the series, it will not be split.
Just like unnumbered or '.5' chapters, it will be counted in the total chapter and volume count.
however, since it is still releasing and has no data listed, it's up to you whether to mark it or just use your list note to signify it as read