SELECT f_id FROM files WHERE formats  =  "mp4" INTERSECT SELECT f_id FROM song WHERE resolution  <  1000
Find the id of songs that are available in mp4 format and have resolution lower than 1000.
[Schema (values) (types)]: | music_1 |  genre : g_name (text) , rating (text) , most_popular_in (text) | artist : artist_name (text) , country (text) , gender (text) , preferred_genre (text) | files : f_id (text) , artist_name (text) , file_size (text) , duration (text) , formats (text) | song : song_name (text) , artist_name (text) , country (text) , f_id (text) , genre_is (text) , rating (text) , languages (text) , releasedate (text) , resolution (number);