Table:
text: Tobu 8000 series
History
1963-1981
Fiscal_year _2_car_sets _4_car_sets _6_car_sets _8_car_sets Total_vehicles
1980 73 45 32 16 646
1981 75 47 33 16 664

Question: What is the smallest 8-car set if 4-car sets are 47 in Tobu 8000 series history from 1963 to 1981?