If Season 4 is a novel, Episode 4 is the climax, but Without "The Nina Project," the finale’s emotional beats would fall flat. It is slow, brutal, and necessary.
The episode explicitly links Henry Creel to Vecna. While the name "Vecna" won't be fully confirmed until Episode 7, 4x5 provides the smoking gun. Henry was Brenner’s first subject. "One" was Vecna. Eleven didn't kill a monster in the lab; she created the void that allowed the monster to escape into the Upside Down. Stranger Things 4x5