Thoughts... Metallica's "Master Of Puppets" was released on March 3rd, 1986. Now, Stranger Things 4, takes place in 1986, around mid-March / Spring Break.
So, Eddie Munson, learned "Master Of Puppets" in about 2 weeks without guitar tabs or utilizing the information super highway known as the internet.
Conclusion, Eddie is either a Guitar God or somehow 'Stranger Things' ties into 'Back To The Future' and he used the time machine to travel time, learned the guitar tab, and traveled back in time to play the guitar tab and save the world... we have yet to learn the full story.
ROCK ON, enjoy your day.
Don't take it seriously and as I was told once...
"Just let art flow, man."