Warning: This article contains spoilers for Stranger Things season 4, volume 1.
history of the Upside Down - and its connection to a monstrous villain named Vecna. In Stranger Things, telekinesis can be used to open gateways to infinite worlds, and the barriers between the dimensions were first torn apart at Hawkins when Eleven engaged One in a telekinetic duel in 1979.
Number One has finally returned as Vecna, using his powers to prey upon the teenagers of Hawkins. He is entering their minds from the Upside Down, taunting them with memories of their greatest fears and past regrets, and finally murdering them with telekinesis. To their surprise, the Hawkins gang learned each murder is opening a small Gate to the Upside Down. Vecna's plans remain a mystery, but it's safe to say the Gates are central to them.
Nancy, Eddie, Jonathan, and Robin were briefly stranded in the Mind-Flayer or the forces of the Upside Down to initiate another invasion; they are biding their time.
The Duffer brothers have claimed to have had many elements of Stranger Things planned from the start, and this idea dovetails rather well in notes discussed in the original Stranger Things Bible. Originally called Montauk, it describes events on a far greater scale than has been shown in the Netflix series so far. It describes the Upside Down spreading across a town like a cancer, manifesting in an ever-increasing number of ways. "Electrical fields will be disrupted. Strange fungi will grow on structures and people. A heavy fog will drift in from the Atlantic. The temperature will plummet. Food will rot. Gravity will fluctuate. People will glimpse bizarre entities in their homes and businesses." The Upside Down has indeed been associated with some of these phenomena over the course of Stranger Things - but the entire town of Hawkins has yet to be affected. Chillingly, that section of the Stranger Things Bible ends with a simple question: "If people can disappear... can an entire town?"
The story of Stranger Things season 4, volume 1 has indeed attempted to draw everybody from Hawkins into the chaos. For the first time, the supernatural events taking place in Hawkins have happened in plain sight, before witnesses who have gone public. The town is panicked and fearful, currently targeting their own youth in a moral panic over Dungeons & Dragons. If this theory is correct, though, things are only going to get worse for Hawkins in Stranger Things season 4 volume 2.