First, as promised, the plot of this new season will be more fully to Hawkins. On the contrary, it is on the side of Russia that the story will move in order, in particular, to discover the advances of the russians in the topic of the World Upside down, and their individual research/experiences.
Second, and this is the most important point of the video, this shift in Russia, we will also find a well-known face : Hopper. The one that we thought was dead after the last episode of season 3 is finally alive and is revealed to be the famous prisoner whose scene post-credits of the time was referring to. It is not yet known how he was able to survive the explosion, what will be his role in future episodes, or how Eleven will manage to find it, but he looks in shape and it is already sufficient.