Heat the oven to 400 degrees. Grease a deep, lidded roasting pan or an oval 8-qt. dutch oven with 1 teaspoons oil. Rinse the chicken and pat dry. In a small bowl, mix the garlic, oregano, remaining 3 teaspoons of oil, salt and pepper. Rub the mixture all over the chicken. Tie the chicken legs together with kitchen twine. Tuck the wing tips under the back. Put the chicken into the pan and add the broth, vinegar, and olives. Sprinkle with brown sugar. Cover and bake 1 hour, then scatter the potatoes and prunes around the chicken. Cover and bake 30 - 45 minutes longer, stirring the potatoes once, until the drumsticks move easily, the juices run clear when a thigh is pierced with a sharp knife and the potatoes are tender.
With a slotted spoon or spoons, remove the chicken, potatoes, prunes and olives to a deep serving platter. Pour the liquid left in the pan into a 4-cup measure. Skim off and discard the fat. Serve the juices with the chicken.