This seems rather simple for 5 kyu?

Nice kata, thanx :)

It wasn't really clear to me, that you only unflatten sub-arrays if you meet them in the array, not if they are "taken". You might want to clarify that - or maybe I'm just slow. (Makes nicer code that you don't have to, anyway - so that's nice ;) )

I too had made a brute force for the 4x4. Which I generalized, and then thought I could use for this one. Well, that was going to take a while... So I optimized, and could easily solve the first example. But the next ones, not so much.

So back to the old drawing board - but that was just so much more satisfying when I found a clean way to do it.

Yes, I really liked this one. Good job. Even if I thought it was going to be a piece of cake after the 4x4, and wasn't :)