I wouldn't hesitate to take a four year old who enjoys water activities.
They will have a ball swimming in the Grand Reef with the stingrays and the tropical fish, they will love grabbing a big noodle and floating around the lazy river, they can watch the otters and monkeys play while swimming around the Freshwater Oasis, they can feed the fish in the aviary, and there's some awesome sand beaches for building sandcastles when everyone needs a little "dry" time.
There are TONS of lifeguards on duty, so you never need to over-concern yourself about safety (beyond the customary precautions and good habits while in the water).
Many, many of the food options are very kid-friendly, and they will love being able to snack on whatever they like, whenever they like (think unlimited ice cream bars

).
Wetsuits and vests are available in a wide range of small sizes so you will have no problem finding attire to properly fit your little one.
I have also heard that if the rest of the family does the dolphin swim, they will invite the youngest family members (who are not old enough to participate) into the water for a family portrait with the dolphin. Can't confirm 100% as it has never happened to me personally, but I have read other reports of it from other visitors here on these boards.