Overall hugely engaging, but in the cold light of day there are a lot of issues: all the stuff in Russia, whilst fun, was pointless and really credebility-stretching.
Yeah definitely the weakest of the plot strands.
The problem with the Russian plot, was I never had any sense that Hopper was in real danger. He was always going to get back to 11, otherwise why not just leave him dead? The whole thing was kind of a pointless divergence.
This show had suffered from diminishing returns in seasons 2 and 3, but volume 1 of season 4 was a big improvement. There were echoes of Nightmare on Elm Street there, with the monster going after depressed kids instead of sleeping ones. They even went as far as getting Robert Englund in.
Volume 2, however was a bit of a mess with its pacing and editing. The teenagers were getting hentai'd for about an hour of the finale's run-time and the ending was a dragged out schizoid mess with too many false climaxes. Did Peter Jackson direct this?
Vecna/Henry Kreel/01's exposition did nothing but make him less scary. I would have preferred if 11 discovered his backstory in a wee vision quest or something. It would have given her something to do other than cry and have angry psychic tantrums.
The Master of Puppets bit was goofy but cool, or "metal" if you prefer. Hopper swording up the demogorgon was fun.