In a medium-size bowl, combine flour, baking powder, sugar, and salt; set aside. In another medium-size bowl, beat egg whites just until stiff, moist peaks form. In another medium-size bowl, beat yolks lightly; stir in milk and butter, and blend well. Pour liquid all at once into dry ingredients; beat until smooth. Fold in beaten whites. Bake in a waffle iron according to manufacturer's directions.