Coat a 9 x 5-inch nonstick loaf pan with fat-free cooking spray and dust with all-purpose flour.
Combine in a medium bowl the cake flour, baking powder, nutmeg (optional), and salt.
In a large bowl, using an electric mixer on medium speed, beat the margarine until light, (approximately 30 seconds). Slowly beat in the sugar substitute and sugar until light and fluffy. Beat in the vanilla, lemon, almond and butter flavor extracts. Beat in the egg yolks, one at a time, beating for 30 seconds after each addition.
With a spoon, stir in the flour mixture alternately with the sour cream.
In another medium bowl, using clean beaters, beat the egg whites until soft peaks form when the beaters are lifted, about 5 minutes. Gently fold the egg whites into the batter.
Spoon the batter into the prepared pan. Bake until a toothpick inserted in the center comes out clean, 55 to 65 minutes.
Cool in the pan for 10 minutes. Invert onto a rack and cool completely.