In that post-movie tease, Scott Lang enters the Quantum Realm in order to collect healing particles to help Ghost, but gets stranded down there after Hank Pym, Hope van Dyne and Janet van Dyne are blinked out of existence. Ant-Man and the Wasp takes place directly after Captain America: Civil War and before Infinity War (the MCU has a lot of wars, huh?), right up until the mid-credits scene that is.
The mid-credits scene is the one that has the most significance to the future of the Marvel Cinematic Universe, because it shows Scott Lang fiddling about inside the Quantum Realm, which the film tells us is a very unsafe place. Ant-Man and the Wasp is the first Marvel movie to hit theaters after that massive cliffhanger as well as the Avengers: Infinity War post-credits scene, which hinted at the long-awaited arrival of a certain superhero.
While any temporal journeying could be the result of the Time Stone now in possession of Thanos, it could also come from Ant-Man's inter-dimensional travels in the Quantum Realm that leads into a time vortex.
An unprecedented cinematic journey ten years in the making and spanning the entire Marvel Cinematic Universe, Marvel Studios' "Avengers: Infinity War" brings to the screen the ultimate, deadliest showdown of all time. He's ready to come back up, and starts the countdown. only Janet, Hank, and Hope aren't receiving his messages. At one point, Scott hears silence from his allies, leaving him trapped in the Quantum Realm. That includes major heroes such as Spider-Man and Black Panther (who we know will be back).
What happens: Back at Scott's house, everything is chaos. Obviously, part of Avengers 4 is going to have to deal with Scott either getting out of or going further into the Quantum Realm. What if the time vortexes are Scott's only way out of the Quantum Realm? But since the film is out, numerous questions we've had since the release of Avengers: Infinity War have been answered - especially the one about how Ant-Man connects to Avengers 4.
"The Quantum Realm is the good news and the bad news in this movie".
Avengers 4 will arrive in May, 2019. It may even be out of Thanos' reach, or at least his awareness.