In March 2017, Kreisberg confirmed that the main villain for the fourth season would not be a speedster, like the previous three seasons. Executive producers Aaron and Todd Helbing also mentioned that there would be less time travel in the season, with Aaron saying, “We like playing with the timelines and the different time periods and future and past. For now, I think we’re going to focus on the present.” In June 2017, Clifford DeVoe / The Thinker was reported to be the main antagonist of the season. He was first hinted in the third season episode “Abra Kadabra” when the titular villain mentions him among the Flash’s greatest enemies, and again in the season finale “Finish Line” when Savitar mentions facing DeVoe but states that the Flash has not dealt with him yet. At the series’ San Diego Comic-Con panel, the speculation was confirmed, with Todd Helbing saying, “With three Speedsters in a row, this year it’s the fastest man alive against the fastest mind alive.” He added that the writers were “making a conscious effort this year to get the fun quotient back up”.
Regarding the possibility of Wally West taking on the mantle of the Flash in Barry’s absence, Keiynan Lonsdale said that “It’s not something that’s on my radar. I feel as though there is so much story to tell and Wally has so much growing to do… We need to see how these character realistically get to where they are headed for.” With the action picking up six months after season three, Kid Flash and Vibe are left to protect Central City, with Carlos Valdes saying “the team is sort of clutching at straws to keep the city together,… there is a unanimous feeling amongst the remaining members that it’s just not the same without Barry… So in light of that, Cisco does whatever it takes to get his friends back.” Valdes also confirmed the return of Cisco’s more lighthearted joking manner. Candice Patton explained Iris West’s larger role within Team Flash as “a way of distracting herself,” adding that “[Iris] is forced to deal with that anger and resentment and abandonment by kind of focusing on protecting Central City… We’re seeing a very different Iris, almost a very hardened Iris.” Kreisberg also hinted at some major growth for Iris this season, and noted that “the season premiere is all about Iris and Cisco, not Barry”.
In July 2017, Sterling Gates, DC Comics comic book writer, joined as a member of the fourth season’s writing staff. At the Television Critics Association press tour in August 2017, Pedowitz stated that the fourth season of The Flash is “going to try to find the lightness… of the Barry Allen of the first two seasons,” and said that the show was likely “done with Speedster villains”. 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.
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. There’s a lot that Iris experienced that Barry didn’t when he was gone, and it’s really [about] the fun and the emotional component of making up that time when they weren’t together.” Helbing also explained that the Speed Force “let [Barry] deal with all of the baggage [from the past three seasons]. When he comes out, he’s sort of left that all behind.” That month, Kreisberg also confirmed that Barry and Iris will be getting married this season, 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.” He added, “The two of them with the therapist is some of the funniest stuff we have ever done on this show, but it also leads to a deepening of their relationship.”