Preheat oven to 375 degrees F (190 degrees C). Lightly grease a 9x5 inch baking pan.
In a bowl, mix the self-rising flour, brown sugar, baking powder, salt. Pour in the beer, and mix just until moistened. Transfer to the prepared baking pan. Top with the melted margarine.
Bake 45 to 55 minutes in the preheated oven, until a toothpick inserted in the center comes out clean. Cool on a wire rack.
(Note: If you don't have self-rising flour, you can make it by adding 1 1/2 teaspoons of baking soda and 1/2 teaspoon of salt for every 1 cup of self-rising flour used.)