Set a large oven-safe skillet over medium-high heat and add 2 tablespoons olive oil. When the pan and oil are hot, place chicken in the skillet skin side down. Cook undisturbed for about 5 minutes, or until the skin is nicely browned. Turn chicken skin-side up and cook for 3 minutes on the other side.
Transfer the chicken to the oven and roast for 20 to 25 minutes.
In a medium bowl, toss the grapes with 2 teaspoons olive oil, sugar, ½ cup sherry vinegar, salt and pepper. Scatter the grapes with their vinegar sauce over and around the chicken and roast until the chicken is cooked through and the grapes are lightly caramelized, 20 to 25 minutes.