Angel food cake gets its characteristic loft from beaten egg whites, otherwise known as meringue, and that's it -- no other leavening agent such as baking powder or soda is needed. You'll want to beat the sugar into the egg whites gradually to achieve the stable structure, and then to gradually fold in the four to avoid deflating them. Let cool completely before unmolding (or it will fall out naturally if you use a classic tube pan and suspend it on its legs or over a bottle, with a wire rack underneath).