In the season 4 clips, we're taken to the laboratory where Dr Brenner performed his experiments on children, last seen in flashbacks in seasons one and two.
A sinister clock ticks in the background; kids chase toy cars around a rainbow track (remember the rainbow room from season two?); someone's playing chess; a ball tumbles through pins on a pegboard; a Magic Eight Ball says "signs point to yes"; there's some spooky child laughter; toy bricks; everything's very echoey and lit in the institutional greeny-grey we've come to expect of bad experimental government agencies. The kids all have shaved heads and hospital gowns like Eleven used to in series one.
We're clearly back in time because we see Dr Martin Brenner (from behind, but we would know that grey bouffant anywhere) stalking the corridors, and as Matthew Modine's voice tells the children that, "We have something very special planned for you today", the camera zooms ominously in on a door marked '11'. The trailer ends with the voiceover saying, "Eleven? Are you listening?" and young Eleven's eyes suddenly open wide in shock.