Hi Staci,
Thank you so much for visiting the Disney Parks Moms Panel with your question. We on the panel love helping guests to plan magical vacations!
You cannot add your 4-year-old to a room that has already reached the maximum capacity as listed for any given Walt Disney World resort room. Thus, if the capacity of the room that you are considering is six guests, you cannot add a 4-year-old as a seventh guest. Guests can add an additional child who is under 3 years old to the capacity of a room; however, since your child is 4 years old, you cannot do this. Let me offer some thoughts on room configurations that could work for you.
You could always book two rooms that sleep up to four guests each. This would allow plenty of space for all of the guests in your party.
Another option is to take a look at
Disney's Deluxe Villa Resorts. The two-bedroom villas at these resorts will sleep 8 or 9 guests, so you will again have plenty of space. These villas also include a full kitchen, living area, and washer and dryer right in the unit, which are great! My family loves having these comforts of home when we travel. A couple of my favorites of the deluxe villa resorts are
Disney's Old Key West Resort, for spacious accommodations and a laid-back feel, and
Bay Lake Tower at Disney's Contemporary Resort, for a sleek and modern feel and the ability to walk to Magic Kingdom.
I hope that these thoughts are helpful for you, Staci! Please do let us know if you have any further questions.