I would say give legoland and universal a miss with a four year old. You simply won't have time. Generally I'd plan for two days in each of the four disney parks and then the disney waterparks and then some down time. If you're staying on site its lovely to do a morning at a park, go back for a laze around the pool and then pop back to the parks at night for fireworks food etc.
We did discovery cove when DS2 was four. He loved playing on the sand making sandcastles and feeding the rays. He was too small to swim with the dolphins but they let him come into the water to have his photo taken.
We had a nice day but it is very expensive for what it is particularly given he can't do the dolphin swim.
Personally I wasn't impressed with Sea World for that age group. You get to see the sea creatures at Epcot anyway so the novelty factor isn't there and the rides aren't aimed at little ones. Ours had the most fun playing in their shorts in a little area of water spurters.
Easter is busy at Disney but probably a better option than the summer just because of the heat/queue combination. October is the best time IMO.