Given @Gary-Campbell-zdjeg7d comment, wondering just when volume 5 will start posting becomes far less a sign of impatience.
I, too, await the next volume from JNC.
But I have faith it will appear.
I'm aware of two titles they have ceased publishing.
The first...the license was revoked by the Japanese publisher; at the same time they pulled all remaining copies off the Japanese market. The publisher was distancing themselves from the author, for good reason.
The second one... OK, while in general once JNC licenses something they don't consider sales as a factor in continuing the license, there are limits. Sales really were that bad.
After-School Dungeon Diver falls into neither of those categories.
So it's some combination of license negotiations and scheduling staff, and that can be a weird combination.