Comment.
      joins("INNER JOIN (SELECT m1.id,m1.topic_id, COUNT(m2.id) as c FROM multimedias as m1 LEFT JOin multimedias as m2 ON
        m1.topic_id = m2.topic_id  AND m1.id < m2.id
        WHERE m1.topic_id IS NOT NULL GROUP BY m1.id, m1.topic_id HAVING c < 4
        ORDER by m1.topic_id, c desc) as m3 ON m3.id = multimedias.id").
      where([ "multimedias.topic_id IN (?)", topics_id ])

Comment.
      joins("INNER JOIN (SELECT m1.id,m1.topic_id, COUNT(m2.id) as c FROM multimedias as m1 LEFT JOin multimedias as m2 ON
        m1.topic_id = m2.topic_id  AND m1.id < m2.id
        WHERE m1.topic_id IS NOT NULL GROUP BY m1.id, m1.topic_id HAVING c < 4
        ORDER by m1.topic_id, c desc) as m3 ON m3.id = multimedias.id").
      where([ "multimedias.topic_id IN (?)", topics_id ]).
      last(4)