As far as having an experience that sets a fun mood for the rest of the day, I'd say Jungle Cruise or Pirates. But with a 4 yr old, I think your best bet is to hit Fantasyland first thing and ride Dumbo. Then do Peter Pan, Pooh, and It's a Small World, before heading over to Adventureland or Frontierland. This way, you're 4 yr old will see that rides are fun and not scary and she'll be more prepared for rides like Pirates, HM and Splash.