In a large saucepan, combine the potatoes, corn, celery, onion, salt and pepper. Add water to just cover vegetables. Bring to a boil; reduce the heat. Cover and simmer for 10 minutes or until the vegetables are tender. Drain and stir in 3 cups of the milk. Stir the flour into the remaining cup of milk with a wire whisk until smooth. Stir into the corn mixture. Bring to a boil; reduce the heat. Simmer for 1 minute. Stir in the sour cream and ham.