The official Stranger Things season 4 trailer provides a legitimate reason why Eleven (Millie Bobby Brown) left Hawkins. Now in the care of Joyce Byers (Winona Ryder) after Jim Hopper's (David Harbour) apparent death, Eleven relocated with the Byers to California, according to a previously released teaser. Though the revelation of their move came at the end of season 3, the group's new whereabouts emerged as a major mystery ahead of the trailer debut.
Toward the end of Stranger Things season 2, Hopper became Eleven's adoptive father as her name was officially changed to Jane Hopper with Sam Owens' (Paul Reiser) help, continuing his guardianship role throughout season 3. After Hopper seemingly sacrificed himself during the Battle of Starcourt, Joyce stepped in to look over Eleven, as shown in the Stranger Things season 3 finale. Still believing Hopper had died, Joyce, Eleven, Will (Noah Schnapp), and Jonathan (Charlie Heaton) packed their belongings and put Hawkins in the rearview. It seemed like the group was on the search for a new start, but the season 4 trailer added another element to their exit, particularly Eleven's reason to leave Hawkins.
During the Stranger Things season 4 trailer, a key ally in the form of Dr. Sam Owens returns. He seemingly arrives in California to warn Eleven about the conflict between their primary dimension and the Upside Down. It's also revealed that Owens was the one who relocated Eleven and the Byers family "far from Hawkins," suggesting he did so to keep them safe. Owens then tells her "a war is coming" before warning that Eleven's friends in Hawkins are "in the eye of the storm," reiterating the belief that Hawkins will remain the primary site of any conflict, which is why it was important for Eleven to get as far away as possible in the first place.
It was also important to get Eleven away from any Russian connection. Since the Soviets used Starcourt Mall as a secret Russian base for their Gate experimentation, they likely received security footage of Eleven using her powers. Though Eleven realized she lost her powers after taking a piece of the Mind Flayer out of her leg, she still managed to use what powers she had to propel a model car into Russian agents to save some of her friends still hiding in the mall. It would not bode well for Eleven if she remained in Hawkins with the Russians knowing her location. Even more dangerous is the theory that Dr. Martin Brenner (Matthew Modine) is working with the Russians in their pursuit of opening the Gates in Kamchatka and Hawkins, which happens to be the only location of a successful Key to the Upside Down.
Brenner pretended to be a father figure to Eleven during her time as a test subject in Hawkins Laboratory, but he was nothing other than a monster. In his eyes, Eleven betrayed his mission when she escaped the lab. With Brenner's expected return in Stranger Things season 4, it would make sense to learn that Brenner has connections to the Russians. Owens may have caught wind of this, giving him more of a reason to relocate Eleven and the Byers. There's also a chance Owens has intel regarding Hopper's survival and capture. This means they could try to interrogate Hopper for the truth about Eleven's powers and experience with the Gate, giving Owens all the more reason to keep her hidden.
It's important to remember that Stranger Things season 4 takes place in 1986, a time when it would still be generally difficult to find someone who has picked up and relocated, especially with the help of someone like Dr. Owens. Still, by the sound of his warnings, Eleven will need to come out of hiding if she truly is "their only chance" with the impending war. Battling Demogorgons and the Mind Flayer was difficult enough, but a new Stranger Things villain in Vecna, in addition to threatening Russians and possible Brenner, means the Hawkins crew would need all the help they can get, meaning Eleven's absence from Indiana will likely be short-lived.
Part 1 of Stranger Things season 4 debuts May 27, 2022, on Netflix.