Hi Jennifer!
How wonderful that your son will be going to Walt Disney World for his 4th birthday.
I would recommend making reservations for a character dinner which features Mickey Mouse. My favourite is
Chef Mickey's at Disney's Contemporary Resort. While characters are always subject to change, Mickey is usually there, along with Minnie Mouse, Goofy, Pluto and Donald Duck. This is one our my family's favourite character dining experiences, and we make reservations here on every trip.
You can make your advance dining reservations
online or by calling (407) 939-3463. I would recommend making your reservation as soon as possible, as this is a very popular dining experience.
You can order a personalised custom cake by calling the Cake Hotline (407) 827-2253. They require 48 hours notice and they can deliver to Chef Mickey's for you (or any other Walt Disney World dining location).
I hope your son has a magical birthday celebration with Mickey!