She is the only character in any of the four posters looking back directly at the viewer, and the poster with Max shows her in the thick of the action in the Creel House. This shows that she has returned to the lab in search of her powers or something else that will allow her to save the world from the Upside Down. We might be able to get some answers before the first volume of season 4 is released later this month, if we look closely at the trailer, posters, and episode list.
The villain of the Upside Down, the evil of that dimensions incarnate, is finally introduced in the trailer.
She was tortured, abused, and trained to use her psychic powers as a child, and has returned to the lab where she was tortured, abused, and trained to use her psychic powers.
Even though Max and Eleven, two adorable best friends, were torn apart by Joyce's good-faith decision to move Eleven and her sons to California, or as far from Hawkins as they could get without leaving the country, this has happened.
According to the trailers for the first volume of the fourth season of the show, Max, played by Fear Street actress Sadie Sink, now has mind powers, while Eleven, played by Enola Holmes, still does not. He was killed by the mind flayer after the piece of the monster that was inside Eleven's leg was returned to the central unit. We know from the trailer and the episode list that she sits at his grave at least once and reads him a letter, followed by expressions of how much she wishes he were still alive.
The piece of the mind flayer took Eleven's powers when she ripped them from her body. When he sacrificed himself, when Eleven lost her powers, and when childhood innocence slipped away, she was struggling with life. The change happened after Eleven pulled a piece of the mind flayer from her open leg wound. Billy, played by Dacre Montgomery, was held by Max.
While we hope that Max will have her own story to tell and triumphs that are separate from Billy, it certainly looks like her first experience with psychic or Upside Down-powered abilities takes place at his grave. Billy was the main squeeze for the entire season. It's obvious that the studios are showing us less than they actually are. Due to the distance, they don't see each other as much, so why has the power switch continued? Even if he is dead, it still has a connection to his mind and history. There must be a connection between the two points that resulted in Eleven losing her powers and Max gaining them. This tells us that Max will have a big part in this new season, and it all leads back to Billy.
Max will learn to wield her powers so that they can team up if Eleven finds her powers restored. The nature of what we see leaves us terrified of what will happen in 1985. The villain might have something to do with Billy, according to Dacre Montgomery.
We don't know where Eleven's powers have gone or why they went to Max. In front of her shocked friends, we see her levitating off the ground.