It is unlikely that the teen group is full for 1/21/06 (you know it isn't since they will let you add at a higher category.)
Many 9s only hold 3 people. Despite the fact that the 4 year old will sleep in your bed,
DCL lists your cabin as holding only 3 people. You might consider changing to a lower cabin if there is a 10 or 11 available that holds 4. I'd suggest a 10 since with a teen you'll appreciate the split bath.
Go to the disneycruise site and put in your dates with your party of 4--essentially pretend that you are making a ressie. See what comes up as availability. If they show it, you can book it!
Ok, I just did that for you. DCL shows availability for a party of 4 in categories 4,5,6,7,8,9,10, 11, and 12. Of course, we know that booking 4 in a 12gty gets you an automatic upgrade to an 11 since no 12s hold 4.
Is your TA one who books a block of cabins and then sells only out of their block? I'd hassle them....DCL has plenty of space for you as of this moment. Your TA may not have the cabin you want, but DCL does! You should be able to book the 9 and just add the extra child fee. Just to be sure I'm right, I checked for 2 adults, a 4 year old and a 14 year old. And I'm not showing your cruise as "almost full." In fact, January is one of the lightest times of the year and one where we typically get great deals.