What is Avengers 4 about?
That’s the real question posed by the very existence of Infinity War: Now what? Surely half of our most profitable heroes didn’t really dissolve into nothing. In the Infinity Gauntlet comic books, before Adam Warlock can set everything right in the universe, Nebula manages to steal the Gauntlet from Thanos and pulls back events by 24 hours. It’s a bit of a cheap move plot-wise -- "just undo everything" -- but it worked on the cosmic scale of the Infinity Gauntlet story.
In the Marvel Cinematic Universe, the only way to reset Gauntlet shenanigans would be to possess the Gauntlet and use it against Thanos. Because of the specific wording of the Infinity War showdown between Thanos and Tony Stark, it seems likely that Tony could make a play for the Gauntlet when they locate Thanos’ vacation home. However, Avengers: Age of Ultron showed that Captain America was almost worthy enough to lift Thor’s Hammer, so Cap wearing the Infinity Gauntlet to save the universe is also a possibility. Which makes sense, as those two characters are the backbone of the Avengers story.
Complicating factors have already been established in Infinity War: 1) Doctor Strange implies that Tony had to live and Thanos had to get the time stone in order for the Avengers to actually "win" in that one-in-multiple-millions future scenario. As far as we know, we’re on that winning path, and Tony Stark's survival is the crux of the plan, 2) the Soul Stone requires the sacrifice of someone you love before you can wield it. Tony’s got marriage plans, and Cap’s the self-sacrificing type, so that whole issue could lead to a Civil War redux.
Even if Avengers 4 takes the comic book way out and has an Avenger wish the entire universe into the past, it probably still leaves The Asgardians, Heimdall, Loki, and Gamora as permanently dead, and chances are good that Vision would need to be actually separated from the Infinity Stone, like Shuri attempts to do in Infinity War, in order for him to survive. But ultimately, everyone who flaked away is coming back... in a little over a year.
Sebastian Stan gave an interview where he cited being in a scene with both Hank Pym (Michael Douglas) and Janet Van Dyne (Michelle Phieffer), and that did not happen in Infinity War, basically confirming that The Winter Soldier meets up with the original Ant-Man and Wasp. That means they’re all somewhere, even if "somewhere" is a Quantum Realm or an alternate timeline.