Clockwork Orange reference
Spoilers be ahead, matey...
In Episode Four, when Rublev and Zelenski are betrayed and end up on a desolate spit of land, Zelenski jumps into the drink, thinking his daughter's stuffed animal is left on the motorboat that is leaving them. Rublev stops him by pointing out that they still have the animal with them, and Zelenski swims back to the dock. Rublev extends his hand to pull him up out of the water, and begins to pull him out.
Then the female KGB agent who has been tracking them suddenly appears on the dock, and they both stare at her in frozen horror. Rublev drops Zelenski back into the water.
Referencing the same scene in 'Clockwork Orange', where Zelenski/Dim falls backward into the water in horror after having his hand slashed by his fellow Droog, Alex.
It comes and goes in just a moment, but I enjoyed this nudge and wink from the director.