The season is produced by Berlanti Productions, Warner Bros. Television, and DC Entertainment, with Andrew Kreisberg and Todd Helbing serving as showrunners.
Grant Gustin stars as Barry, with principal cast members Candice Patton, Danielle Panabaker, Carlos Valdes, Keiynan Lonsdale, Tom Cavanagh, and Jesse L. Martin also returning from previous seasons, and are joined by Neil Sandilands.
[1] Barry, Oliver, Sara, Alex, Martin and Jax wake up in a Nazi concentration camp on Earth-X wearing power dampeners.
They learn that the only way back to Earth-1 is through a temporal gateway in a research facility, which Schott plans to blow up to strand Dark Arrow (Oliver's Earth-X doppelgänger) on Earth-1.
Oliver disguises himself as Dark Arrow to infiltrate the facility and discovers the Nazis have a contingency plan, a militarized timeship called Wellenreiter.
Barry and Ray battle the Freedom Fighters' Red Tornado to stop it from destroying the gateway while the rest of the team enter the facility to reactivate its portal.
Executive producer Andrew Kreisberg said on this, "The great thing about our dear friends at The CW and Mark Pedowitz picking the shows up as early as they did has allowed us to start building the schedules for next season.
[67][68] In November 2017, Kreisberg was suspended from his role as executive producer and showrunner on The Flash over allegations of sexual harassment.
[83] Later in the month, Kreisberg confirmed the return of Harry Wells from Earth-2, saying that "season 4 for Harry is really realizing what he's missing in his life and what it is he needs to become a better, more complete person, and so he's going to be going on a fairly epic emotional journey this season that is tied to the Thinker's plan," in addition to confirming the introduction of a new version of the Wells character.
[84] In September 2017, Helbing noted "There's a lot of love in the air this season," and compared Barry and Iris' reunion to "somebody going off to war for six months and coming back.
"[85] That month, Kreisberg also confirmed that Barry and Iris will be getting married this season,[86] while also revealing that the pair will be going to couples therapy since "[they] keep comically clashing inadvertently because [both are] used to being in charge, and so they wind up going... to work through it."
[95] 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.
[34] 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.
[100] 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.
[102] Prior to that, Arrow actress Emily Bett Rickards appears as Felicity Smoak in the fifth episode of the season.
[62] In July 2017, cast from the series appeared at San Diego Comic-Con to promote the season, where exclusive footage was shown.
[103] 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.
"[105] 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.
Club, Scott Von Doviak gave the finale a B+, praising the light tone and character moments, while adding that "None of this can make up for the long stretches of mediocrity this year, but at least it doesn't leave me with a bad taste in my mouth.