(SPOILERS FOR S4E9)
So, in the final moments of the episode (or, well, the climax, I guess) when everything's going to shit and the four gates are tearing Hawkins inside out- or in this case, upside down, I guess. I didn't notice this until the second time I watched the episode, but the four gates all meet at the center of town- the Library. Now you may think, "big deal, the library is in the center of town, that's where the gates were headed." and you'd be right.
HOWEVER.
Let's retrace our steps back to Season 1, when Will gets kidnapped from Upside Down Castle Byers by Demogorgy. The Library (or at least the Upside Down counterpart) was where the Demogorgon held Will captive after capturing him.
Cue my mind being blown.
Now, this may just be a huge coincidence, and to be honest it probably is. But is it symbolic as hell? Absolutely.