Preheat the oven to 450 degrees F.
Sift together flour, sugar, baking powder and salt. In a separate, large bowl, mix the sweet potatoes and butter. Add the flour mixture to the potato mixture and mix to make a soft dough. Then add milk a tablespoon at a time to mixture and continue to cut in. Turn the dough out onto a floured board and toss lightly until the outside of the dough looks smooth. Roll the dough out to 1/2-inch thick and cut with a biscuit cutter. Place the biscuits on a greased pan and coat tops with melted butter. Bake for about 15 minutes. (Watch your oven: If the biscuits are browning too fast, lower the temperature.)