Tyler Hoechlin's Superman won't be returning in Supergirl season 4. Introduced in Supergirl season 2, Hoechlin's version of the Man of Steel has become a firm fan-favorite. This latest season has seen The CW expand his world significantly, introducing Elizabeth Tulloch as Lois Lane and Jon Cryer as Superman's arch-nemesis, Lex Luthor.
The "Elseworlds" crossover saw Superman and Lois Lane announced their engagement - and revealed that Lois is pregnant. They headed offworld to visit Argo City, the last surviving Kryptonian city, so that Lois could give birth under a red sun. Superman asked Supergirl to take over and protect the Earth in his absence. Fans remained convinced that he'd be returning sooner rather than later, though, especially with the news that Cryer would be playing The CW's Lex Luthor.
According to ComicBook, during a screening of this weekend's episode of Supergirl, producers Robert Rovner and Jessica Queller were asked whether Superman would be returning. Both Queller and Rovner answered, "not this season." The precise wording presumably indicates that - unsurprisingly - they are planning for Hoechlin to appear in Supergirl season 5.
While this news will disappoint viewers who were hoping to see Superman face off against Lex Luthor, it does make sense; Supergirl has always been wary of using Kara's cousin as a plot device to resolve overarching story arcs. The introduction of Lex Luthor is sure to be a major part of Supergirl season 4, and there's even been speculation that Luthor is the true mastermind behind the problems that have plagued Supergirl. If that's the case, it's a far more effective plot for Kara to face Luthor without Superman's help.
Meanwhile, assuming Superman will return to Earth in Supergirl season 5, it's reasonable to assume enough time will have passed for Lois to have given birth. That will propel the Man of Steel into a fascinating new place as a character, one that's never been explored in a DCTV series before. More concerning, though, is the fact that Superman will presumably be arriving in time to participate in the "Crisis on Infinite Earths" event. This promises to be the biggest event in the history of the Arrowverse; it's been foreshadowed since the pilot episode of The Flash. In the comics, "Crisis on Infinite Earths" led to the death of Supergirl; it's possible that the Arrowverse version of that story will see Superman pay the ultimate price in order to save the Multiverse, leaving Lois to bring up her baby without a father.