Chief Jim Hopper is alive, as revealed in the "Stranger Things" Season 4 teaser. However, he may not be the same Hopper fans have grown to know and love.
Fan theories are never-ending when it comes to the storyline and plot for the upcoming season. Recent speculations involve the possibility that the Hopper shown in the trailer is a clone, with the Mind Flayer responsible for the cloning.
According to Screenrant, the cloning may have happened after the explosion at Starcourt Mall in Season 3. A previous theory claimed that Hopper escaped the explosion at the underground Russian lab when he was thrown into the Upside Down. However, Hopper may have been cloned by the Mind Flayer to serve its purpose, which is to lure Eleven.
Cloning Hopper would make it easy for the monster to get its revenge on Eleven. She interfered with its plan to conquer the human world in Season 3, when she prevented Billy Hargrove from executing any more of its plans. Eleven and Hopper share a close bond and once she finds out that he is alive, she would not hesitate to approach him. This would make it easy for the Mind Flayer to kill Eleven when given the chance.
The question though, is how a cloned Hopper ended up in Russia as seen in the "Stranger Things" Season 4 teaser. It is supposedly likely that he ended up in a safe zone during the explosion and the Russians took him in as a prisoner.
The Russians would also have their own agenda for keeping him and not feeding him to the Demogorgon. Hopper knows a lot about the Upside Down and the monsters that live there. He is also Eleven's father figure and his connection to her would benefit them. Thus, they would likely also clone him to get their hands on Eleven so they could use her powers.
The idea of having a cloned Hopper in "Stranger Things" Season 4 is not far-fetched given how the Netflix show references 1980s movies. John Carpenter's "The Thing," which tells the story of an alien life form that takes the form of the human it kills, was heavily teased in the series (from the poster in Mike's basement and Lucas' fascination with Coke because of the film).