The information is found here.
http://www.seaworld.com/seaworld/fla/index.html
Single day tickets are 47.95 and children are 38.95. You won't get any discounts doing single day tickets, but it will be worth it, your daughter will be so happy. The park opens at 9am, and the Key West dolphin area is open at this time. Its located on the left side of the park as you enter, not far from the entrance. Get there as early as you can so you can buy your tickets before a line forms.
On feeding the Dolphins... They have a window where you purchase the fish. They have a clock that says when the window opens. Get in the line before the window opens, because a long line forms. Buy 4 cups at a time (maybe more) because they go fast, once they run out, the window closes. Bring lots of money, your daughter will want to keep feeding. Have her stand on the wall while you go get the fish because it gets crowded, you gotta wiggle your way into a spot. Everyone stands along the wall and the dolphins come up to those who have fish. They can see who does and who doesn't have it. You can still get some touches by standing next to someone who has fish. But the person with the fish gets all the attention. As people run out of fish, they leave, and the crowd is less. If she still has some fish, then she can feed them without all the other people around, but it's hard to do this. Most people run out. An hour or two later, they'll open the window again, and the entire thing repeats. In between feeding sessions, we went underground and looked at the dolphins thru a glass wall, it's so cool.
Yes, 4 hours is enough time to visit Sea World, do the dolphins, see a couple shows, have lunch, and then leave for the airport. And DC is right next to SWF, you can even see it by going up in the sky tower ride. There is an extra charge to go up in the tower ride, but it's worth it, I think it's like $4 a person. You'll also be able to see the Renaissance Resort located across the street from Sea World, this is the hotel you wanna stay in when you come back and do a DC trip.