Ant-Man and the Wasp may be very much a self-contained film but even though the focus is on its titular characters there are strong connections to the rest of the Marvel Cinematic Universe -- and it may have some major implications for Avengers 4.
Before we get too deep into breaking down what Ant-Man and the Wasp means for the fourth Avengers film you need to know that there will be major spoilers for Ant-Man and the Wasp beyond this point.
The primary focus of Ant-Man and the Wasp is the quest on the part of Hank Pym (Michael Douglas) and Hope van Dyne (Evangeline Lilly) to locate Janet van Dyne (Michelle Pfeiffer) within the Quantum Realm and rescue her. Scott Lang/Ant-Man (Paul Rudd) is instrumental to this as his own brief time in the subatomic world has given him a connection with Janet that lets them locate her. While things don't go quite as smoothly as Team Pym would hope -- there wouldn't be a movie if there weren't some complications, after all -- the heroes ultimately prevail. Janet is brought back from the Quantum Realm only to be turned to dust along with Hank and Hope when Thanos snaps his fingers, leaving Scott trapped in the Quantum Realm where he was sent to collect energy.
It sounds like a major bummer, but the shocking loss of most of Team Pym may have some major ramifications for what comes next and not just in terms of attempting to undo Thanos' snap. For starters, Ant-Man and the Wasp makes a point to establish that the laws of time and space are different in the Quantum Realm. While it's a case of tell instead of show in the film the telling hints that time travel is possible. This is further implied when Janet cautions Scott to avoid a "time vortex" before sending him for quantum energy. If he engages with the vortex, they can't save him.
Janet's warning is a curious one. While decades spent in the Quantum Realm certainly gave Janet a lot of time to learn how it works her warning about the vortex seems uniquely specific. While the reasoning behind Janet's warning is not explained in the film, it's not impossible that she has encountered them herself and ended up deposited into another time. If the vortexes really are a conduit to other pockets of time, then Scott is in a perfect position to go back and undo the events of Infinity War -- or at least figure out how it's possible and team up with the other remaining heroes for an epic rescue mission.
There's also a fan-theory that suggests that Scott being trapped in the Quantum Realm at the exact time of the snap is the one and only outcome Doctor Strange saw in which the heroes win. Given that we've known Stephen Strange is aware of the existence of the Quantum Realm -- the Doctor Strange film briefly depicts it as one of the planes of existence he's aware of -- Janet's mention of time vortices seem all that much more important. When you also consider that the fan theory that Thanos also went back in time to a farm on a lush, thriving Titan after the snap may have been confirmed by some newly-released Infinity War concept art, the concept of time is even harder to ignore.
However, even if time travel and those time vortices end up not playing a direct role in Avengers 4, the MCU isn't likely to be done exploring the Quantum Realm anytime soon, opening up a whole new world for the heroes to explore going forward
Ant-Man and the Wasp is currently in theaters. Captain Marvel will follow it in March of 2019, with Avengers 4 coming in May of 2019 and Spider-Man: Far From Home slated for release on July 5, 2019.