Hi Moses!
Thanks for dropping by the Moms Panel with your question!
You can certainly use your fourth day to go back to any of the parks you'd like. I'd probably go back to
Magic Kingdom too - it's my favorite park; followed closely by
Epcot (awesome food, LOVE strolling through the
World Showcase).
If you're able, consider adding the
Park Hopper option to your existing passes. There will be an upcharge to doing that, but it really is worthwhile...and a great value! You'll be able to come and go freely to as many parks as you'd like on any given day. You could spend the day at one park, and if you want to dine at another, you can do that with no problem!
Please let me know if you have any questions. I'm happy to help! Wishing you a magical day and a pixie dusted vacation!