From Redditor u/Split_Bananas:
I was recently doing a rewatch of Stranger Things Season 4 and noticed some major foreshadowing in the first episode. I'm convinced the Duffer Brothers have already told us what will happen in Season 5, from events during the first episode of Season 4, as they also foreshadowed the finale in the same episode.
During the intro to episode 1, Eleven is catching the audience up on what has been going on in California. One thing she mentions is that Will has been painting a lot, speculating that it may be for a girl. She then says she thinks there is someone he likes, because he has been acting weird.
I think it's obvious now that he was acting weird because he would be seeing Mike for the first time in eight months and he was struggling with his sexuality and his feelings for his longtime friend. [When he reveals] to Mike this painting, which depicts Mike as the leader of their group of friends, [he uses] it as an analogy to express Eleven's feelings for Mike. The moment after this, as Will looks out the window and is holding back tears, suggests that Will was talking about his feelings for his friend, not Eleven's.
Another event that was foreshadowed was what would happen to Hawkins at the end of this season. As Chrissy was waiting on Eddie Munson in the woods to sell her [drugs], she experienced her second hallucination caused by Vecna. During this hallucination, we are [shown] the grandfather clock. As the clock hits its fourth strike, symbolizing Vecna's fourth kill, the clock splits from each of the four corners and meets in the center. The center of the clock is then split open and spiders come rushing out. This is what happens to Hawkins once Vecna gets his fourth kill and the gates become connected.
This most clear sign to support my theory happens during [the] Hellfire Club's campaign finale. Eddie Munson was delivering the details of their battle with Vecna and says that Vecna's cultists turn to the players and remove their hoods, only to have the players recognize most of them. I believe these will be the people that Vecna has killed. I believe it was Dr. Brenner talking about how Vecna doesn't just kill his victims, he absorbs their power and their soul. Eddie goes on to say that there is one cultist they don't recognize, his skin shriveled and desiccated, but something else too.
He is missing his left arm and his left eye.
All of these characteristics of Vecna's body are results from his battle with Steve, Robin, and Nancy.
Eddie continues the campaign and [the] Hellfire Club agrees to fight Vecna to the death. Vecna goes on the kill the entire party except for two players. With two rolls left, Dustin rolls once and misses, while we also hear the sound of the grandfather clock. Erica rolls and gets a critical hit; the gang wins, but with severe loss.
The Duffer brothers using D&D to foreshadow as they always do.
In summary, Season 5 will consist of Eleven and the gang… having to face Vecna and his army, with the army being familiar faces… possibly Max too? And of course they can't end the series without Will dealing with his conflicted feelings. I chose to mention the event with Will, although there are a few other examples of foreshadowing in the same episode.