I've been using a base overworld by Azurik since Hack 2, and I've been using 1 or 2 of its submaps for each hack. This is why you'll see an entire custom overworld in Hack 4, and you'll see the same thing in Hack 2 and Hack 3. Doesn't have any other significance. Unused submaps may be used for future Hack hacks, though.
Fantastic as always! Really fun and incredibly creative. One complaint I'd have is that in the level where you can jump in midair, sometimes the B button won't press even if there's more jumps left. I don't know if there's anything you can do about that, though. This was a blast to play through and (whenever it may come) I can't wait for Hack 5!