Anna,
Kind of hard to know how to work out four parks in three days, but here it is.
- April 18 - Arrival Day
- April 19 - Magic Kingdom (AM) / Hollywood Studios (PM)
- April 20 - Non Park Day
- April 21 - Animal Kingdom (AM) / Epcot (PM)
- April 22 - Magic Kingdom (all day)
On the 19th you have conflicting evening options. The Magic Kingdom's evening parade and fireworks are real
Must Do! activities. But Hollywood Studios has Evening Extra Magic Hours, which is a lot of extra hours on a short trip.
So while I always recommend doing the Magic Kingdom first, especially on your first trip, you might spend that morning at Hollywood Studios, and the Magic Kingdom in the afternoon/evening. Your call.
As to the 21st, I have you at Epcot in the evening for a great meal and the best fireworks in all of Walt Disney World.
Finally on the 22nd I have you back at the Magic Kingdom, as there are more rides there than any other park, more to do, more to see. And with Morning Extra Magic Hours, you can get one more hour in.
Have a magical vacation!