Combine flour, baking powder, sugar, and salt in a large mixing bowl.
Add the melted butter, milk, and vanilla extract to the dry ingredients. Use an electric mixer at medium speed to combine the ingredients.
Separate egg yolks from egg whites. Add all four egg whites into a separate medium mixing bowl. Add the egg yolks to the bowl with the batter and combine well.
Beat the egg whites with an electric mixer until the egg whites are stiff.
Use a large spatula to gently fold the stiff egg whites into the batter. Don't overwork.
Pour the batter into the preheated waffle iron. Cook the batter according to the manufacturer's instructions.