Preheat oven to 425. Sift flour, baking powder, sugar and salt into a large mixing bowl. Transfer to a food processor. Cut butter into pats and add to flour, then pulse 5 or 6 times until the mixture resembles rough crumbs. (Alternatively, cut butter into flour in the mixing bowl using a fork or a pastry cutter.) Return dough to bowl, add milk and stir with a fork until it forms a rough ball.
Turn the dough out onto a well-floured surface and pat it down into a rough rectangle, about an inch thick. Fold it over and gently pat it down again. Repeat. Cover the dough loosely with a kitchen towel and allow it to rest for 30 minutes.
Gently pat out the dough some more, so that the rectangle is roughly 10 inches by 6 inches. Cut dough into biscuits using a floured glass or biscuit cutter. Do not twist cutter when cutting; this crimps the edges of the biscuit and impedes its rise.
Place biscuits on a cookie sheet and bake until golden brown, approximately 10 to 15 minutes.