Avengers 4's title is officially Avengers: Endgame, which fits with our earlier theory about what the title would be. Avengers 4's title has been kept secret for so long that many Marvel fans already believed it must have some kind of incredible significance. "End Game" is in fact a significant phrase, because shortly after Doctor Strange voluntarily handed over the Time Stone to Thanos (ostensibly as a trade for Iron Man's life), he said cryptically, "We're in the End Game, now."
Even though Avengers: Infinity War and Avengers 4 directors Anthony and Joe Russo were asked point-blank if the fourth movie's subtitle is ever spoken aloud in Infinity War, and Joe Russo firmly said, "No", they did just that. Sometimes, directors lie when they're trying to keep a secret.
It's little wonder that fans have been feverishly speculating about Avengers 4, given that Avengers: Infinity War ended on a massive cliffhanger. After successively overcoming the combined force of the Avengers and the Guardians of the Galaxy, cosmic supervillain Thanos collected all six Infinity Stones and used the Infinity Gauntlet to wipe out half of all life in the universe. Believing that things were finally balanced, Thanos then effectively retired to live a quiet life, leaving what's left of the Avengers (basically just the original line-up) to deal with the fallout and try to find a way to undo it.
Obviously they will find a way to undo it, because the casualties included characters like Black Panther and Spider-Man, who we know have their own solo movies coming up (in fact, we just learned the title of Spider-Man's next solo movie). The question is - how will they do it, and what did Doctor Strange mean when he talked about the Avengers being in the "End Game"?
The key clue in Avengers: Infinity War is the scene in which Doctor Strange says that he has explored 14 million possible futures, and only found one in which the Avengers win - and he didn't look particularly happy about it. Based on this, and on Doctor Strange telling Tony Stark that "there was no other way," the most popular fan theory is that the one possible future where the good guys win requires letting Thanos win first. Essentially, Doctor Strange had a bigger "endgame" in mind. If the movie is called Avengers: End Game, then that more or less confirms the first half of the theory... but the second half is where it gets tricky. Now that Thanos has won, how can how our possibly undo that?
Based on the set photos from Avengers 4 that show scenes from the Battle of New York, it's generally assumed that the Avengers will somehow use time travel to undo Thanos' finger-snap and restore the universe to the way it was. One fan theory posits that Doctor Strange somehow tampered with the Time Stone, which Thanos used to turn back time after Scarlet Witch killed Vision and destroyed the Mind Stone. If that use of power was somehow corrupted, it could have created a crack in time (or something equally comic book-y) that the Avengers can exploit.
Then again, since Infinity War's post-credits scene saw Nick Fury calling upon Captain Marvel for help, we may have to wait for the release of her solo movie next year before we have all the clues about how the Avengers will get out of their current predicament. But if Avengers 4 really is the endgame for the Marvel Cinematic Universe... what comes next?