What is the average weight of Japanese cars with 4 cylinders that were produced from 1975 to 1980?
Japanese car refers to country = 'Japan'; with 4 cylinders refers to cylinders = 4; produced from 1975 to 1980 refers to model_year BETWEEN 1975 AND 1980; average weight = avg(weight)
SELECT AVG(T1.weight) FROM data AS T1 INNER JOIN production AS T2 ON T1.ID = T2.ID INNER JOIN country AS T3 ON T3.origin = T2.country WHERE T2.model_year BETWEEN 1975 AND 1980 AND T1.cylinders = 4 AND T3.country = 'Japan'