If you are talking of 100% entertaining your 4yr old, then yes, go with sjaneyamat, Disneyland will be the place. Though consider the kid wont be able to go on a lot of attractions due to age/height restrictions.

If you are looking for a place in general, and a 4yr old will be just as excited with the Star ferry and a trip to Victoria peak etc, then the YMCA (Salisbury Hotel)where we stay is ideal. Not expensive, has in door heated swimming pool, few hundred meters walk from Star Ferry terminal, 2 minutes from under ground.

Or best yet, split between the two, spend some time in Disney, and some down town (YMCA).