In a large bowl, beat the butter until creamy. Add the sugar and beat until combined. Beat in the egg whites, 1 at a time, then beat in the flour. Refrigerate the dough until firm, if necessary. Divide the dough in half and transfer to 2 large sheets of plastic wrap. Shape the dough into 2-inch-thick logs, and wrap in the plastic. Refrigerate until firm, at least 2 hours.