It is being reported that the new additions will consist of three teenagers and one adult. The series is reportedly seeking a trio that will include a "metalhead to an entitled jock to a character that sounds an awful lot like the twin of Fast Times at Ridgemont High stoner Jeff Spicoli." TVLine reports that the adult character will possess characteristics which will become a focal point in the upcoming season.
Following the events of the Season 3 finale, there was concern among Stranger Things fans regarding the fate of Jim Hopper after it appeared that he sacrificed himself in order to save the town of Hawkins. However, there was a glimmer of hope in the finale’s mid-credits scene which hinted that “the American” detained in a Russian military laboratory could be Hopper.
Even though production isn’t reportedly set to begin for another few months, the groundwork for Stranger Things Season 4 has been laid with the brief teaser released in September.
With Eleven joining Joyce, Will, and Jonathan Byers as they leave Hawkins in the Season 3 finale, it was assumed that if there were a fourth season, the portal to the Upside Down in Hawkins would bring them back together or the world created by the Duffer Brothers would expand to parts unknown. The teaser stating that "We're not in Hawkins anymore" suggests it will be the latter. Where exactly remains a mystery.