The 5th book (another short story anothology) has reached a critcal point, it's more than one third completed. Oddly enough the story I finished was the first short story I started from the series. It was originally going to be handled as a flashback in the third book but I felt like the storyline didn't further the overall story and was simply a unique adventure. It helps explain some of the details that appear in the third book without having to actually slow down that book with a tangent.
From the standpoint that you question when these anthologies end up in your grasping, clawing hands the answer is... I'm not yet sure. At first I was expecting I'd put out both books individually, but the more I work on them the more I see them as a complimentary pair so there is a strong chance they'll simplly end up getting released at the same time.
We'll see where my whimsy carries me as I continue to work on these stories and filling in some of the background that was previously removed to keep the pace up and keep the focus strong.