As well, the big bad for Season 4 will be Clifford “The Thinker” Devoe, whose non-speedster ways won’t be very effective in the Speed Force-related shenanigans that Rebirth is centered on.
Admittedly, porting The Flash: Rebirth’s plot onto the live-action Flash series isn’t the most sensible route for Season 4, since the TV show has already incorporated (and dispatched) most of the big villains involved in the original story.
In the comic story, Reverse-Flash creates a negative version of the Speed Force that affects those connected to it, so perhaps the premiere will present a flipped-up take on the Speed Force that explains how Barry is able to escape after being voluntarily trapped inside for at least six months.