Main cast members Grant Gustin, Candice Patton, Danielle Panabaker, Carlos Valdes, Keiynan Lonsdale, Tom Cavanagh, and Jesse L. Martin return from previous seasons as Barry Allen / The Flash, Iris West, Caitlin Snow / Killer Frost, Cisco Ramon / Vibe, Wally West / Kid Flash, Harrison Wells, and Joe West, respectively. Cavanagh will portray Harry Wells from Earth-2 at the start of the season, without ruling out the possibility of introducing another incarnation of the Wells character later on. Teasing the arrival of a new Harrison Wells, Cavanagh said, “We’re going to start with Harry and then find a way, I think, to try and get a different version of Wells in there, be it some version of Harry or whatnot as the season’s gaps present themselves.” Also returning from earlier in the series are Jessica Camacho as Gypsy, and Anne Dudek as Tracy Brand. Tom Felton, who joined the cast as Julian Albert in the third season, was revealed not to be returning to the series, although there is a possibility he could return if the storyline demanded it and Felton’s availability permits. Julian’s absence will be addressed “pretty quickly — there’s a reason why he’s no longer with the team,” said Helbing. 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, will reprise 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 seasonlong battle with the Flash in order to fix all that he deems wrong with humanity; while Kim Engelbrecht was announced as The Mechanic, Devoe’s right hand and a highly intelligent engineer who designs devices for him; and Danny Trejo as Breacher, a bounty hunter from Earth-19 and the father of Gypsy. 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 / Blacksmith, who operates an underground black market of metahuman supervillains.
Filming for the season began on July 4, 2017, in Vancouver, British Columbia, and is set to conclude on April 21, 2018.
In August 2017, series composer Blake Neely and Nathan Blume began to compose the music for the fourth season.
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 will begin with Supergirl and a special airing of Arrow on November 27, 2017, and conclude 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.” Emily Bett Rickards will appear as Felicity Smoak in the fifth episode of the season.
The season is set to begin airing on October 10, 2017, on The CW in the United States, and on CTV in Canada.
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.com’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!”
“(#401) “The Flash Reborn””. The Futon Critic. Retrieved September 21, 2017.
Saclao, Christian (July 29, 2017). “‘The Flash’ Season 4: Is DC Villain Hazard Appearing In Episode 3?”. International Business Times. Retrieved August 9, 2017.
Saclao, Christian (August 12, 2017). “‘The Flash’: Tom Cavanagh Is Directing Another Episode Of The CW Series”. International Business Times. Retrieved August 12, 2017.
Abrams, Natalie (August 21, 2017). “The Flash adds Katee Sackhoff as DC Comics villain”. Entertainment Weekly. Archived from the original on August 21, 2017. Retrieved August 21, 2017.
Gustin, Grant (August 21, 2017). “Eclipse got us like… (that’s our director, Laura and Andy in the back doing the classic hand shade. Don’t worry. I don’t think he looked at the eclipse like that) Also, don’t worry about the dribble on my shirt. It’s supposed to be there. You’re gonna have to watch episode 405 of @cwtheflash to find out what’s going on with that little, juicy tidbit.”. Instagram. Retrieved August 21, 2017.