I had to get my four-year-old up at 4:30 am for a trip in March. And by "get up", I mean, I took him out of bed and put him in the car still in his pajamas. That's one of the benefits of being four! (Or three.) I'd either do what pp said and dress her in comfortable clothes with elastic waist shorts the night before, or just take her to Disney in her pajamas. When you get there, one of you can take her to the restroom to change. Plus, then you have a change of clothes (or pjs) in case they get wet or ice cream on them or something. Snacks in the car if she likes to eat first thing, otherwise I'd bring a banana and a Ziploc of dry cereal to eat while you're in line.
Also, be prepared that she just might not fall asleep earlier than her normal schedule. You can try wearing her out the day before (go swimming, run in the park, etc.) but that can SERIOUSLY backfire if they still don't fall asleep early and then are extra, extra tired the next day. I'd put her to bed at the earliest end of her normal, cross your fingers, and hope for the best. Will you have a stroller that reclines and has a sunshade? Even non-nappers are prone to crashing in a stroller on vacation.