Development The series was renewed for a fourth season on January 8, 2017, In November 2017, Kreisberg was suspended from his role as executive producer and showrunner on
The Flash over allegations of sexual harassment. Sonny Wells of Earth-24 and H.P. Wells of Earth-25. Also returning from earlier in the series is
Jessica Camacho as
Gypsy. Kreisberg also confirmed that he had put on hold the planned return of
Violett Beane as
Jesse Quick due to the high number of speedsters on the show, but did not rule out a possible return in the future.
Britne Oldford, who previously appeared as
Shawna Baez / Peek-a-Boo in the
first season and the web series
The Chronicles of Cisco, reprised her role in the season premiere. In July 2017,
Neil Sandilands was announced to be cast as Clifford DeVoe, a
metahuman genius who embarks on a season-long battle with the Flash in order to fix all that he deems wrong with the world; while
Kim Engelbrecht as
Marlize DeVoe, DeVoe's loyal wife, a highly intelligent engineer who designs devices for him; and
Danny Trejo as
Breacher, a bounty hunter from Earth-19 and the Gypsy's dad. Also in June, the series was looking to cast a role that "should put the visual effects department to the test", with the role later revealed to be
Ralph Dibny / Elongated Man, a metahuman with the ability to stretch his body to superhuman lengths and sizes.
Hartley Sawyer was cast in the recurring role that July as the fast-talking private investigator who after discovering his abilities will help Team Flash solve one of Central City's greatest mysteries. The character was previously mentioned in the first season, as one of the fourteen people who seemingly died as a result of the particle accelerator explosion. The next month,
Katee Sackhoff was announced in the recurring role of
Amunet Black, who operates an underground black market of metahuman supervillains.
Filming Filming for the season began on July 4, 2017, in
Vancouver, British Columbia, and concluded on April 21, 2018.
Kevin Smith returned to direct an episode in January 2018.
Music In August 2017, series composer
Blake Neely and Nathaniel Blume began to compose the music for the fourth season.
Arrowverse tie-ins 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. 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 in the twenty-second episode. ==Marketing==