The 10% is only for the 7 night cruise. There is no percent off for a 4 night, but I believe there will a $100 onboard credit (the $200 is for the 7 night).
Wanting to cruise so soon, if I were you, I would look at prices from TAs. If you go to the Disney website, you will get their quote online. Then just consider there will be a $100 onboard credit too. Now it COULD go up in the next month, but this will be a good place to start.
Then get some quotes. IF you get a quote that is more than a $100 less than the
DCL quote, then it would be better to book with that TA (and do it sooner rather than later because people are starting to book up the spring break cruises).
A good place to get a quote from a TA that is highly recommended on this board (I haven't used them, but friends of mine have and they are pleased) is with All Seasons Travel. You can just click through their pages and get a quote:
http://cruises.allseastravel.com/c/5/5738/1528933_5738.htm#3-6-2005
You should also get a quote from
Dreams Unlimited (that you will do by sending an email, but there is no obligation):
http://www.dreamsunlimitedtravel.com/
I wouldn't be surprised if one of them were more than a $100 less than DCL is because March 2005 dates have been out for a long time and that is a high travel time.
Good Luck!
DJ