Just finished watching Season 4.
First of all, talk about a loving tribute to 80's slasher horror... Strong Nightmare On Elm Street 4: Dream Warriors vibes. Not a ripoff but a definite homage. And I was definitely here for it.
Storywise, it was all lore, all the time and, again, that's what I'm talking about.
What worked for me: the "Goonies never say die" vibe of the main plot. When Eddie and Dustin connected in the final episode, that's Stranger Things at its finest.
What didn't work for me: The USSR B plot. It started silly and proceeded to ridiculous and then culminated in ridonkulous. Hopper with a Conan sword at the end is exactly what Teen Titans GO! were mocking in "Nostalgia Is Not A Substitute For An Actual Story."
Soundtrack was fairly off-the-hook. Philip Glass' "Prophecies" is starting to become a musical trope, right up there with Barber's "Adagio For Strings." However, they still know how to tug heartstrings just right, even with the anachronistic use of Moby.
Overall, I liked that they really focused on (a) a definite and coherent plot and (b) relationships. Whether it was the Eddie/Dustin dynamic or Steve and Nancy just charming the pants off me in the Winnebago, it's a nice return to form.