Heat oven to 400 degrees F. Spray cookie sheet with no-stick cooking spray. Combine flour and 2 tablespoons of the brown sugar in medium bowl; mix well. Cut in shortening with pastry blender or fork until mixture resembles coarse crumbs.
Combine milk, sweet potato and sour cream in small bowl; blend well. Add to flour mixture all at once, stirring just until moistened (if dough is too dry, add additional milk 1 teaspoon at a time, until dry ingredients are moistened). Drop dough by 1/4 cupfuls onto prepared cookie sheet. Sprinkle with remaining 2 tablespoons brown sugar.
Bake 15 to 20 minutes or until biscuits are golden brown. Immediately remove from cookie sheet. Serve warm.