-- QUERY 4
SELECT `plane`.`type`
    FROM `airport`

INNER JOIN `flight`
    ON `airport`.`code` = `flight`.`departure_airport_fk`

INNER JOIN `city`
    ON `airport`.`city_name_fk` = `city`.`name`
    AND `airport`.`city_nation_fk` = `city`.`nation`

INNER JOIN `plane`
    ON `plane`.`id` = `flight`.`plane_fk`

WHERE `city`.`name` = "Denver";