Hi Mona:
Your child is bound to have a wonderful vacation. There aren't really any services I recommend that you spend your money on other than a stroller. There is so much walking and activity at Walt Disney World that you and your four year-old will appreciate the convenience of a stroller, even if your child hasn't used a stroller at home in years. You can
rent strollers in the Parks, but my family uses a local Orlando stroller-rental service, orlandostrollerrentals.com. They are familiar working with Disney World guests and conveniently deliver and pick up the stroller right from your Resort.
While not a necessity, you may consider making a reservation for your child at a
Children's Activity Center. They are generally open during late afternoon and evening hours for potty-trained children 3-12 years old. Any Guest can make a reservation at the Childrens Activity Centers, you do not have to be staying at the particular Resort of the Center. You can use these hours to recharge while your child has fun with other kids in a supervised environment.
My best advice is to follow your child's pace and enjoy. Remember to carve out some afternoon quiet time, which will help you enjoy the Parks being refreshed.