How many female actors acted in the movies of year 4?
Female actors mean that a_gender = 'F'
SELECT COUNT(T2.actorid) FROM movies AS T1 INNER JOIN movies2actors AS T2 ON T1.movieid = T2.movieid INNER JOIN actors AS T3 ON T2.actorid = T3.actorid WHERE T3.a_gender = 'F' AND T1.year = 4