Cruise pricing is very tricky it depends on a lot of factors. The pricing for a cruise in June (if you bought it recently) would be different than the pricing for the same cruise a year later (if you buy it a year in advance). To illustrate, we booked our Cat 6 July 9 West Coast (verandah) cruise 14 months in advance and were charged around $4000 (for 2 adults and 1 under 3 DD). A couple of months ago, I started to wonder if we could save a couple of bucks by downgrading to a Cat 10, if it was available. The quote came back higher than the price we had locked in for the Cat 6 (needless to say we decided to keep our current reservation).
You don't say whether your hoping to cruise soon or later or what time of year you want to go (September is the most cost effective), but I can tell you that for a family of four you need Cat 11 or higher since I don't believe that the Cat 12's hold 4 people. This also affects pricing.