Make sure to set a pace he can handle. The parks can be a lot of walking for a four-year-old. Try the water parks, they have special kids' play areas and loads for him to do. Plan mid-day breaks to wind down. Be sure to teach him to go to a CM if they get separated [CMs wear white name tags*]. Take a picture of him with a digital camera every morning in whatever he's wearing that day, if she gets lost it'll help the CMs find him.
* Any guest can buy a name tag that looks like you work there, but only CMs have white ones. I have a name tag and often get people asking directions and telling their kids to "find someone with a nametag like this man" if they get lost. Then they get mad at me when I point out that any child-molester or freak can readily buy a coloured name tag and that CMs and only CMs have white ones! :shrug: