Oh, I see where your problems are: restricted to the most expensive time and needing a stateroom that fits 5 people. One option might be to get two inside staterooms (which would also give you more space in total) than one 4D which seems to be the lowest category available right now. I would suggest to find a travel agent that specializes in Disney trips (there are several ones to be found online, Kingdom Konsultant is one of them and is a sponsor of these boards, but I have no personal experience with them) as they will be able to help you find the best option for your needs.
However, in my opinion, if you can't get the price down, a 3-night cruise is not worth 5000 $. It will be over before it started. See it this way: for the 70 hours you will be on board, you pay 1000 $ each. Even with all the free food and soda this is just far more than I would ever be willing to pay. Especially if I knew that about half of what I am paying is not for the experience, but for the date. Ultimately, everyone has different priorities. So, you really have to figure out if it is worth it for you? Is there a chance that in a few years time you will be able to be more flexible with regard to dates, etc.