Catagories 1 to 3 are obviously very expensive. Catagory 4, your paying for a slightly larger verandah room, but others have pointed out that the verandah becomes practically useless once the bed for the fifth person is down for the night.
My personal opinion is that you will be much more happier if you try for two rooms some how. I don't remember for sure, but the last time this issue was discussed, the ball-park prices were that two inside staterooms were cheaper than a Catagory 4, that a Cat 6 (verandah) and Cat 11 (inside state-room) accross the hall from each other was about the same price as a Cat 4, and a pair of Cat 6 was about 25% more than one Cat 4.