On i-cruise.com they are offering your cruise Cat. 11 at $1293 pp and Cat. 4 at around $500 pp (depends on what cat. 4). So for 2 adults and 4 kids thats about $4600 so I'd say your cost is about right.
But just to make sure, post your request on the Bargain Finders feature on this web site and let the Travel Agents reply with their quotes.