Supergirl season 4 is taking cues from the seminal DC Comics story, Superman: Red Son and having Kara's greatest villain become herself. The Supergirl season 3 finale was more transformative than expected. Reign, was defeated, but nearly everything else changed around that event. Mon-El and Winn both joined the Legion of Superheroes and traveled to the future to fight a new threat, explaining why neither will be a series regular in season 4. Brainiac 5 decided to stay in the present, explaining why his actor Jesse Rath will be a series regular for season 4.
Lastly, and maybe most importantly, a second Kara showed up in snow-covered Siberia in the season's last moments. This Kara was born of Supergirl's aura that escaped during the final showdown with Reign. It's unknown if this new Kara will be the main villain of Supergirl season 4 or not. However, she'll definitely be a huge part what will unfold and her appearing in Siberia is very intentional.
In an interview with TV Line, Supergirl showrunners Robert Rovner and Jessica Queller explained that Superman: Red Son is a huge inspiration for the next season of the series and the season 3 ending teaser. Rovner explained, "For fans of the comics, I think they might recognize that the story we want to tell is inspired by the iconic DC Comics’ Red Son, which told an alternate origin story for Superman."
Red Son is one of DC Comics' many Elseworld stories, which imagine worlds that are similar but different in key ways, to the main DC Universe. The premise of Red Son is simple. It begins with baby Kal-El escaping Krypton and landing in the Soviet Union, not in the middle of America's heartland. Red Son sees Superman became a symbol and ward of the Soviet state. Through Superman and his powers, the Soviet Union makes impressive strides to controlling the entire world. The only serious resistance ends up being the President of the United States, Lex Luthor.
It's pretty safe to say that story won't be replicated exactly in Supergirl season 4 and not just because the President of the United States is an alien played by Lynda Carter and Lex Luthor is a convicted criminal. Like many comic stories the Arrowverse has adapted it's more likely Supergirl will take the core idea of Red Son, a Kryptonian under the control of Russia, and mold it to their own purposes. Rather than chart a Siberian Kara's rise and fall in the Russian government, the main Supergirl will likely come across and face off against her (probably evil) clone.
The concept of there being two Supergirls in season 4 is strange but not unprecedented. The last Arrowverse crossover event, "Crisis on Earth-X," saw Kara come up against an evil Nazi version of herself named Overgirl. Since Overgirl and Melissa Benoist's performance were one of the best parts of the excellent "Crisis on Earth-X," it's probable that she's somewhat behind the decision to adapt Red Son for Supergirl season 4.
Benoist definitely has the acting capability to play season 4's hero and main villain. Hopefully Red Son isn't the entirety of Supergirl's season 4 plans. Supergirl has never been a series to shy away from political discussions or comic book weirdness and Kara facing off against a Russian version of herself would undoubtedly have healthy amounts of both. Yet there's only so much story that Supergirl can explore with Kara seeing an evil version of herself, especially after season 3.
Much of Supergirl season 3 was about Kara exploring her own personal dark side and pulling away from it into the light of optimism. Having a Red Son-inspired Supergirl as the main villain would likely hit on much of the same thematic beats. It would be better if the Red Son story opens Supergirl season 4 or is just a smaller arc within it, rather than being the entire year-long story.
Source: TV Line