In a saute pan heat 3 tablespoons of the olive oil, add the eggplant, and saute until soft and browned, about 10 minutes. With a slotted spoon or wire basket, remove eggplant and and place in a bowl, keeping any remaining olive oil in the pan. In the same pan, add the final 1 tbsp olive oil to the remaining oil, and cook the onion, celery, peppers and mushrooms. Saute until soft, about 4 minutes. Add the garlic, tomato sauce, vinegar, cooked eggplant, sugar, salt and pepper to taste. Bring to a boil, reduce heat, and simmer for 10 minutes until vegetables are tender.