Hi, Julia!
Oh, you're going to love this plan! Yes, there is a version of Disney's Oceaneer Club and Lab on Disney's private island,
Castaway Cay. Therefore, if your 4-year-old is registered, then she's welcome to be checked in and out there, and she'll be familiar with the counselors who are from the ship. Scuttle's Cove -- an extension of the on-board youth program -- has a tented area for activities, and there is a roster of beach activities for that age group. It's included in the price of your daughter's cruise fare.
In addition, Scuttle's Cove now features a new whimsical water play area: a 1,200-square-foot soft wet deck area provides fresh-water fun with an array of pop jets, geysers and bubblers. With organized activities and Monstro's Point—a giant whale-bone excavation site -- the most difficult logistic may be trying to get your little one to come with you and your older child once you're done on the jet skis!
I hope this helps you plan. Have a wonderful day and a magical vacation. Bon voyage!
Andi