Hello, Jaleesa!
Aw! A fourth birthday in Walt Disney World! Your little prince is going to be so excited!
If I were to spend one day in the parks, I would avoid park hopping. While it's a great option for folks who want to spend the morning in one park, but return to a different park for an evening meal, or for those who are staying longer, I think you'd be disappointed that you wasted some of your park time traveling from one park to another. (It can take an hour or so to get from one park to another, depending on transportation, traffic and lines at the bag checks and turnstiles.) I'd stick with one park for you little man's big day.
If choosing between either Magic Kingdom or Disney's Hollywood Studios, you can't beat the magic of
Magic Kingdom. Hands-down, there are so many attractions, parades, shows and the closing fireworks, you couldn't possibly get to it all in one day. For an idea of what's going on in Magic Kingdom on your son's big day, click on the link above, then select "Calendar" from the list on the left. Scroll to the date in December and click on it. There, you'll see the exact times of major events throughout the day at Magic Kingdom, so you'll be able to plan that day much more efficiently.
Big birthday wishes from the Moms Panel to your little prince! Please send more questions as you think of them! We love to help!