This morning the trailer dropped for Avengers: Endgame, the official title of what we've been calling Avengers 4 all this time. As expected, things have sort of exploded as fans begin to pour over the trailer to get a hint of what's next Earth's Mightiest Heroes, and while the trailer is careful to actually tell us very little, it does reveal an interesting story for one of the MCU's "smaller" character. We know that Scott Lang makes it out of the Quantum Realm on his own somehow, and we know that simple fact is really important to the movie.
Scott Lang/Ant-Man was nowhere to be found in Avengers: Infinity War. It's revealed through dialogue that he is on house arrest following the events of Captain America: Civil War. At the end of the last MCU movie, Ant-Man and the Wasp Scott Lang is actually inside the Quantum Realm when "the snap" occurs. Either for this reason, or just random luck, he survives, but the people who were supposed to pull him out of the Quantum Realm are dusted.
Clearly, the Avengers believe Scott Lang is dead. Officially, he appears to be listed as "missing" so it seems that, since they just couldn't find the guy lost in the Quantum Realm, they don't know what happened. Still, with half of the universe dead, they clearly believe that he's gone for good.
This is because the final sequence of the trailer reveals that Scott Lang is, in fact, very much alive. If Scott Lang had simply been missing in action you'd expect Captain America's reaction to Scott being alive would be relief, but that's not it. He's shocked. He assumes he's watching some old recording rather than a live video of Avengers HQ's front gate.
The trailer is telling us that this is important. By making this scene the last one in the trailer, the Russo Brothers are making it clear that Scott Lang being alive, that his escape from the Quantum Realm, is the answer. Somehow, this piece of information is key to the story of Avengers: Endgame.
What's interesting, and possibly the answer to this question, is that, while Black Widow says the shot is from the front door, implying it's happening in that moment, if you look at the top left corner of the shot, it says the video is an "archive" implying that is is an old video as Cap says. This would seem to indicate that, while Scott Lang has escaped the Quantum Realm on his own, he may not have come out at the right time.
This falls in line with many fan theories that have surmised that the Quantum Realm will be the key to the Avengers using time travel as a way to undo the events of Avengers: Infinity War.