In a large bowl, combine the tuna, cheese, onion and celery. In a small bowl, combine sour cream and mayonnaise. Pour over tuna mixture and toss to coat. Spread about 1/2 cup over four slices of bread; top with remaining bread. Butter the outsides of sandwiches. On a griddle or in a large skillet over medium heat, toast sandwiches for 4-5 minutes on each side or until lightly browned.