Stranger Things season 4 could very well be the show's version of Avengers: Endgame, in which it will wrap up every major plot thread just like how the MCU movie concluded Marvel's Infinity Saga. On the one hand, it’s easier to see this grand accumulation of Stranger Things’ expanded universe come to pass if, indeed, season 4 is the last that creators Matt and Ross Duffer plan to make - it would serve as the ultimate climax to the overarching narrative, wrapping everything up in grand style.
On the other hand, though, there’s ample evidence to suggest that this could (and, perhaps, should) still be the case even if Stranger Things season 5 is ultimately produced; the Duffer Brothers have a tendency to not end on big action set-pieces but, instead, smaller, character-focused denouements, such as the Snow Ball dance in season 2 and the Byers family moving out of Hawkins in season 3. In this regard, Stranger Things season 5 could serve as an epilogue, similar to how Spider-Man: Far From Home was an epilogue for the MCU's first saga.
One of the ways that Stranger Things season 4 could resemble Endgame is if all the characters - including those from past seasons - return to defeat an unstoppable threat or to attend a funeral for someone who's been with them since the beginning. For the Avengers, that included minor players such as Happy Hogan, Okoye, Red Skull, Pepper Potts, Thaddeus Ross, Jasper Sitwell, Peggy Carter, and a whole host of others; for the Netflix series, we could be looking at Terri Ives, Suzie, Dr. Sam Owens, Eight, and Murray Bauman, as well as Chief Jim Hopper and Dr. Martin Brenner (the individuals currently believed to be dead).
Such a cavalcade of characters will also necessitate a number of plot threads previously thought to be tangential, at best, to also return and to be integrated more thoroughly into the main narrative. The best example here is perhaps “The Lost Sister”, the infamous episode from season 2 that squarely focused on Eleven, Eight, and their misadventures in Illinois. Those figures (along with, possibly, all the rest of the test subjects from Project Indigo) could be revealed to be an important storytelling facet, just as Marvel integrated the more-or-less isolated Ant-Man films into the main plot of Avengers: Endgame (through the auspices of the Quantum Realm).
And viewers shouldn't be surprised if revisiting all of this material from previous episodes will be done in a strikingly similar manner: Stranger Things’ writers have already demonstrated an affinity for slipping flashbacks into the proceedings, including those that are comprised of brand-new material (such as seeing Joyce Byers and Bob Newby snuggling on the couch); in much the same way, Endgame used time travel to revisit certain periods and locations that were previously separated, rendering the previously standalone Guardians of the Galaxy and Doctor Strange movies into some of the most critical installments of the MCU.
And, of course, all of this will be done in service of ending the ultimate evil in the universe/multiverse, the Mind Flayer - who is the perfect analog to Thanos. In the same way that audiences saw the Avengers fall in the battle against Thanos in Avengers: Infinity War, viewers likewise shouldn’t be too terribly surprised when some of their favorite residents of Hawkins will be forced to sacrifice their lives in order to seal off the Upside Down once-and-for-all. Even if the parallel dimension still manages to creep back in, thanks to the Soviets, in a potential season 5.