Congratulations on you little princess gaining more mobility/independence.
You said she does fine 90% of the time, so to you challenge when planning this trip is going to be to change your focus from the 90% to the 10%. Plan for the ten percent of the time. Through my 14 years as an Elementary / Preschool level special education teacher, I learned a long time ago that for field trips I needed to focus on that 10% to have a successful field trip. The approach works for outing and trips with the family also.
If you plan your trip around the 10% of the time that things are not at their best, then when things are going fine you have an extra bonus. When things go into that 10% you are prepared for it and it should not take away from the trip or experience. Bring her stroller, walker, and other tools she has used over the last year. She does fine with the braces, but prepare for her having a sore under the braces and maybe having to have time without them. With all the walking around Disney, she may just decide it is too much and not put in as much effort as she does at home and school.
You will find many posts with people saying how Disney brought out so many positive elements in their kids and had their kids gain skills due to Disney. This would be wonderful if she came back from Disney with better mobility skills, but don't plan for that. Plan for her having less while at Disney.
I can't tell you the number of times I have gone on trips looking at the 90% and ending up with a kid in the 10% percent. - - Yeah, he is toilet trained 90% of the time and rarely has accidents. Yet on the trip he rarely used the toilet and had several "accidents" (not that you can really call it an accident if the child isn't trying to get to the toilet). - - - Prepare for the 10% and then when the child goes there, it is not a problem for you came prepared for it.
From what little you have said about your

Bring or
rent a stroller, bring her braces and the shoes for the braces, bring shoes that she wears without the braces, and get a GAC. And most important of all, have fun without the disability getting the way of the 4 year old little girl.
