For a four year old:
All the parades, especially Share a Dream Come True and Mickey's Jammin Jungle Parade (which should be the holiday version for your December trip)
Spectromagic.
Wishes.
Fantasmic.
All three of the big shows, Festival of the Lion King, Voyage of the Little Mermaid and Beauty and the Beast.
I know its way more than three but they are all too good to miss! And I would suggest Mickey's Very Merry Christmas Party (you need a separate ticket for this event). Also, check out the wonderful holiday decorations, the monorail resorts in particular have incredible gingerbread houses. That is a great way to take a break from MK, ride around on the monorail, check them out, maybe have lunch at Chef Mickey's. And at Epcot they have Santas from each of the countries. Its a great time to go, especially with a child of that age. You have double magic: Christmas and Disney!