Warning: massive spoilers ahead for Ant-Man and The Wasp! Read ahead at your own risk!
Phase 3 of the Marvel Cinematic Universe has started to wind down, and the final pieces have finally begun to fall into place. That's particularly notable in the end credits sequence for Ant-Man and The Wasp, which seems to hold some massive implications for Avengers 4. More specifically, Peyton Reed's latest Ant-Man film seems to imply that the Quantum Realm's power over time, as well as Janet van Dyne's restorative powers, may play considerable roles in the events of the final Phase 3 movie.
If you have seen Ant-Man and The Wasp, then you already know that it ends with Scott Lang trapped inside of the Quantum Realm following the disappearances of Hank Pym, Hope van Dyne, and Janet van Dyne after Thanos unleashes the power of the Infinity Stones in Avengers: Infinity War. The fact that Scott is inside of the Quantum Realm when the finger snap happens seemingly protects him from the effects of that moment, and his presence inside the Quantum Realm could hold the key to sending him back in time to undo the devastation.
To understand this, we need to remember a very specific piece of information dropped by Janet before he enters the Quantum Realm. She tells him to watch out for Time Vortexes while in the microscopic universe, suggesting that there are wormholes in there that could send him back in time. Set photos leaked from Avengers 4 have already shown that Scott Lang likely goes back in time to The Battle of New York in The Avengers, so the act of trapping him in the Quantum Realm feels like the first step towards bringing that narrative to life.
Then there's Janet van Dyne and the restorative powers that she brings back with her from her time in the Quantum Realm. Though the movie does not entirely explain it, Janet's ability to heal Ghost (and the subsequent harvesting of energy from the Quantum Realm) seems to imply that the Quantum Realm can patch holes in time and space. With that in mind, this could mean that Janet van Dyne is basically Marvel's equivalent of Watchmen's Doctor Manhattan -- a being of immense power who can manipulate reality due to her time trapped in the Quantum Realm. This potential could range from her somehow managing to bring herself back from being dusted, or playing a key role in bringing others back if Scott can rescue her from her apparent death.
This idea is perhaps best exemplified by a recent guest post that mathematical physicist Spyridon Michalakis wrote for The Wrap, in which he explained the infinite potential within the Quantum Realm. Michalakis wrote:
That sounds like some pretty intense science fiction, but it cuts to the core of what makes the Quantum Realm so interesting. It's a landscape full if limitless potential. We have only scratched the surface of what that bizarre universe is capable of, and onething tells us that the manipulation of time and reality could play huge roles in the overarching narrative of Avengers 4.