1. Heat oven to 425 degree F. Arrange a metal folding rack in a 13x9-inch roasting pan. Set aside.
2. Rinse chicken under cold running water; pat skin and cavity dry with paper towels. Discard excess fat. Sprinkle cavity with 1/4 teaspoon each salt and pepper. Place lemon half, 1 garlic clove and 1 rosemary sprig inside cavity. Thinly slice remaining 3 cloves of garlic and remove leaves from remaining rosemary branches.
3. Carefully lift skin away from chicken breast with fingers or the handle of a wooden spoon. Place 2 teaspoons butter, sliced garlic and rosemary leaves under skin. Rub remaining 4 teaspoons butter over entire chicken. Sprinkle with remaining 1 teaspoon salt and 1/2 teaspoon pepper. Tie legs together with kitchen string. Place chicken, breast side up, on rack. Roast 30 minutes. Reduce oven temperature to 350 degrees F and roast 45 to 50 minutes more, until meat thermometer inserted in chicken thigh registers (without touching bone) 180 degree F.
4. Transfer chicken, to cutting board. Cover loosely with foil; let stand 10 minutes before carving. Makes 4 servings.