Remember to get your "First Visit" badge from City Hall.
Playhouse Disney Live is on our list of must-dos. Your 4 year old would especially enjoy it if they're familiar Mickey Mouse Clubhouse, Handy Manny and Little Einsteins-do you get those shows in Australia?
I haven't seen Teacups, Tiki Room or Autopia mentioned yet, my kids love them.
If you're Star Wars fans, don't miss the Jedi training academy (if your child gets picked to participate you will have some priceless picture ops!)
They would also really enjoy stuffing an animal at Build-a-Bear in Downtown Disney-you can dress the animals in Disney costumes.
I think you should also enjoy some Mickey shaped food (pretzels, ice cream bars, rice krispies)
Also, more for mom and dad, you might want to get a paper cut silouette souvenier done of your child on Main Street or a caricature in New Orleans Square- if they'll sit still long enough.