Hi Shelley, welcome back. We had a great time, and it seemed you did too.
Although we started bringing our kids when my 27 year old was younger than your child, it was mostly because we drove down and it took so long, I would have been away from him too long. If you have someone to leave your child with, and you want the older kids to have a great time with you and your hubby, she'll prob be thrilled, not to have to go, if she doesn't enjoy it yet.
Another alternative would be to bring a "nanny", if you have an older niece, or relative that would like to come with you. They could sleep in the children's connecting room, and you and hubby would be able to enjoy night life a few evenings, as well as have your own room, two bathrooms, etc.
As far as fitting in the room, well lots of people have had opinions about that..some put the five in, others don't, since they would have to lie. I don't understand (if you don't mind the closeness) why firecodes would allow a baby in a pen and 4 other people, but not a 3 year old who could easily sleep in the double bed. The pen, IMO is much more dangerous...however, that is Dis's rule : (
We had no problem fitting our grandson in who turned three one year (yes the one eating the ice cream in my sig), along with my then 5 yo granddaughter, my hubby and adult daughter. Disney knew he was turning 3 during the trip, but said as long as he was under the first night, it was OK. We were not at all uncomfortable, and wish we could still do it.
For not much more, you can transfer to POR, where they fit 5 comfortably, and not "break" the rules.