The latest teaser for season four of Stranger Things is officially live, and it gives us a closer look at the dark and twisted Rainbow Room where El (Millie Bobby Brown) spends a portion of her childhood. In the trailer, we see children crawling on the floor over a rainbow and playing various games in a room with a rainbow wall. Enter a figure that looks eerily like Dr. Martin Brenner (Matthew Modine), the manipulative scientist that El calls "Papa" in the first season. Feel familiar? Rainbows are something we've seen before in Hawkins — in case you've forgotten, here's a little refresher on the Rainbow Room in Stranger Things!
Long story short, the Rainbow Room is a room at the Hawkins Lab where Brenner brought children to play. But Brenner's intentions were far from fun and games — he would run tests on these kids as well. As a child, Eleven would play with Eight, or Kali, inside of the Rainbow Room. In a flashback, Terry Ives, El's mother, actually breaks into the lab with a gun and finds El there. She almost retrieves her daughter, but Terry gets caught before she could break out. As a reminder, Brenner had experimented on Terry when she was pregnant with El, which is why El, or Jane, now has psychokinetic powers.
From the teaser so far, we've seen children playing in the Rainbow Room. We can also spot a figure that looks like Dr. Brenner (given the trim suit and white hair) tell children playing in the room that there's something special planned for them. Then, a voice asks, "Eleven, are you listening?" Right now, it's safe to assume that this is just a flashback, but it's very entirely possible for us to see Dr. Brenner again in present day — after all, we didn't technically see him die in season one after the Demogorgon mauled him down!