<P> Filming for the season began on July 4, 2017, in Vancouver, British Columbia, and is set to conclude on April 21, 2018 . Kevin Smith will return to direct an episode in January 2018 . </P> <P> In August 2017, series composer Blake Neely and Nathaniel Blume began to compose the music for the fourth season . </P> <P> In May 2017, The CW president Mark Pedowitz officially announced plans for a four - show Arrowverse crossover event, crossing over episodes of the television series Supergirl, The Flash, Legends of Tomorrow, and Arrow . The crossover, Crisis on Earth - X, began with Supergirl and a special airing of Arrow on November 27, 2017, and concluded on The Flash and Legends of Tomorrow on November 28 . In August 2017, Berlanti noted that it would be "a big life event for a few different people" that would bring them all together for the crossover, adding "There are many life events that happen ." Prior to that, Arrow actress Emily Bett Rickards appears as Felicity Smoak in the fifth episode of the season . </P> <P> The season began airing on October 10, 2017, on The CW in the United States, and on CTV in Canada . Sky One acquired the rights to air the season in the UK & Ireland, airing it alongside the other Arrowverse shows . The season premiered October 17 . </P>

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