From their facebook page:

Poster, May 4th 2015: "I noticed that Hollow Fields volume 4 was removed from your series release list why is that? Did you think that not enough people would buy it or did it just never exist in the first place?"

Seven Seas Entertainment, May 5th 2015: "It'll come out next year. --AA"

We don't know anything about the content, as far as I know. I find this very interesting, since #3 seemed like an ending. I wonder if this will be an epilogue? Sequel?

Thoughts…?