Whole roasted garlic bulbs are a delicious garnish for this recipe, and you can roast them at the same time that you roast the chicken.
Let chicken stand at room temperature 30 minutes. Preheat oven to 425 degrees. Remove giblets and any excess fat from chicken cavity. Rinse chicken inside and out with cold water; pat dry with paper towels. Tuck wing tips under body. Season cavity with salt and pepper; stuff with parsley, rosemary, and 2 lemon halves.
Gently separate skin from chicken with your hands. Spread garlic butter over entire surface of chicken and under skin. Generously season with salt and pepper. Cross chicken legs, and tie ends together with kitchen twine. Transfer chicken to a roasting pan. Arrange remaining 2 lemon halves around bird.
Roast chicken, basting occasionally with accumulated pan juices, until skin is crisp and deep golden brown, and an instant-read thermometer inserted into thickest part of thigh registers 175 degrees, about 1 hour and 10 minutes.
Transfer chicken and lemon halves to a platter. Let chicken stand 10 to 15 minutes. Garnish with rosemary, and whole roasted garlic, if desired.