HEAT THE OVEN TO 450F. Sift the flour with the baking powder, salt and baking soda. Cut in the vegetable shortening until mixture resembles bread crumbs. Stir in the buttermilk with a wooden spoon until just blended. Turn the mixture onto a floured surface, and knead 10 times. Pat the dough into a rectangle 1/2-inch thick. Cut with a 3-inch cutter, and place biscuits close together on an ungreased cookie sheet. Bake in the center of the oven for 10 to 12 minutes, or until golden brown. Serve immediately. Makes about 20. Biscuits can be made a day in advance and covered tightly in aluminum foil. Reheat in a 300F oven for 10 minutes.