HDD is the most fun/memorable meal we've had at WDW (DD was 6 the first time we went and she loved it). We have done 50's, Tusker, CP, CRT, Luau, 'Ohana (bfast & dinner), WCC, BoG - all were good and we liked them (to varying degrees) but nothing like HDD. We also like the atmosphere in Sci-Fi but I don't think a 4 year old would enjoy it much.
It really does depend on what your little one likes, if she likes specific characters then one of the buffets would be better. But if she (and you) like more of a (Disney) party atmosphere with music, dancing and comedy then HDD would be a good one. The food is good too!