We always book a cat 9 room -- and we love it. The cat 9 is the exact same size in the cabin as all rooms up to, I think, a cat 4. The only difference between the cat 9 and the cat 5-7 is the verandah (and the deck); the only difference between the 8 and 9 is the location of the cabins.
We are also a family of 4, and just got off the Magic, and the cabin was fine. Our only problem was that we had too much "stuff" because of the port change, so not all of our luggage fit in the closets. Other than that, we had no room size issues. Of course, our kids are underfoot in a 2500 square foot house; it was the same in a square foot cabin!
Marcy