One thing to consider with a Cat 4 is that one bed drops down from the top of the cabin, acting like a top bunk over the sofa bed. Will the six year old sleep on that? I know my youngest was a little put off by it. Luckily, her older brothers have a bunk bed at home and were used to it.
Also, don't underestimate the value of a veranda, especially for naptime, as someone suggested. My daughter was ill our first cruise - took a four hour nap. I ordered room service and sat and read on the veranda, which was really really nice.
Last cruise, because the kids were older, we did the two cabins. I have to admit, I was a little freaked out by how small they were and the fact that there was only a porthole, which I couldn't open. I was ready to go find a Cat 4 cabin and trade!
When the kids were small, I really enjoyed the Cat 4 and didn't find it too crowded at all. I'm glad we did it. Once the kids were older, the two cabins worked out great too. One cabin would have been TOO tight for us all. Next time, we'll probably do the veranda cabin and the inside cabin across the hall.