<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 . Following the crossover, Katie Cassidy (who has previously appeared on The Flash as both Laurel Lance and Black Siren) appears in the nineteenth episode as Siren - X, an alternate version of Laurel from Earth - X . Arrow actor David Ramsey appears as John Diggle, as well as DeVoe's Diggle disguise earlier in the twenty second episode . </P> <P> The season began airing on October 10, 2017, on The CW in the United States, and on CTV in Canada . The season concluded on May 22, 2018 . 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> <P> In July 2017, cast from the series appeared at San Diego Comic - Con International to promote the season, where exclusive footage was shown . During the panel, a trailer for the season was shown, with James Whitbrook at io9 feeling that despite the "grim" tone, there was "some fun signs of the team coming together to protect the city without (Barry),...Sprinkle in a few wacky things, like, say a goddamn Samuroid ripped straight from the comics, and ladies and gents, you've got a good season of The Flash lined up ." Ben Pearson with / Film felt seeing Iris West deal with the absence of Barry was "a nice change of pace for that character", but anticipated that "Barry (would) be back two or three episodes in at the latest ." Collider's Allison Keene also noted Iris "getting an actual storyline" with Barry gone, and similarly presumed that he "(would) be back in the fold by the end of the first episode" alike to Flashpoint in the third season . She added, "There are some new foes, lots of action, plenty of tech--it's great!" </P>

When will season 4 of the flash be released
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