My grandkids loved Atlantis when they visited this February. They were 2 and almost 3 1/2 when they were there.
Your 4 year old will be limited from several of the water activities because they are only for kids 48" and taller.
However, there are nice kid pools with smaller slides, the beach, the Dolphin encounter (extra fee) and the indoor and outdoor aquariums or habitats.
http://www.atlantis.com/thingstodo/waterpark.aspx
There are free movies appropriate for all ages every afternoon at the Coral Towers part of Atlantis. (Free movies for teenagers and grown-ups usually early and late in the evening. The Harborside Concierge Desk has the movie schedule and can help with anything you need.)
Harborside has cribs and cots that are $10 for the week (IIRR) and can arrange for babysitters. The tariff for babysitting is rather steep but sometime you just have to get away from the kids for a nice, quiet dinner.
Enjoy! Atlantis is for all ages. But, only you can decide if your trip will be for your whole family. ... eom