The first two people in any cabin are charged as adults. In your case, you'd be paying 4 adult fares and one infant fare. Connecting cabins are a good deal if you get a category 10, which is an inside cabin. It's usually cheaper than a category 4, which does hold 5 people. I believe as you go up, a category 9, might be breakeven, and anything above a category 9 would be more than one category 4.