Toss frozen French fries with 1 tablespoon melted butter. Place 1/4 of the fries in a single layer in the center of the first piece of foil. Loosely fold foil around edges, leaving a large hole at the top to allow steam to escape; repeat with remaining fries and foil. Place foil packets on preheated grill. Cover grill; cook 20 to 30 minutes, stirring once, until fries are crispy and baked through.