Blizzard Beach and Typhoon Lagoon both have a kiddie area. Personally, I think Typhoon Lagoon is a lot better for a 4 year old. First, Blizzard Beach has very little shade and Typhoon Lagoon has a fair amount of shade. Second, Blizzard Beach has much bigger and faster rides, which your 4 year old will not be able to go on. And third, Typhoon Lagoon has a huge surf pool which Blizzard Beach does not have. My children prefer Typhoon Lagoon, but I think it is because of their age. In my opinion, Typhoon is better for younger children. Blizzard Beach is great for the pre-teens and teen-agers. It won't be long before my 10 & 8 year old are going to want to go to Blizzard Beach, but in the meantime my 5 year old is thoroughly enjoying Typhoon Lagoon.
Good Luck
D. B. Kelly