At the end of Ant-Man and the Wasp, we see Scott Lang exploring the Quantum Realm when his teammates, Dr Pym, Janet, and Hope all turn to dust when Thanos snaps his fingers. The implication of this was that Scott was now stuck in the Quantum Realm with no one to help him come out. However, the Avengers 4 trailer shows Scott standing outside the Avenger’s facility and asking to be let in. He has his van with him, which contains the smaller version of the Quantum Tunnel. This obviously means that he made it out of the realm, somehow.
During the end credit scene from Ant-Man and the Wasp, Janet warns Scott to stay away from time vortexes while he is subatomic. Considering the fact that these post-credit scenes are quite short, every second of information is crucial and it is doubtful that the creators waste any time with needless information. So, the leading theory on how Ant-Man got out of the Quantum Realm is that he somehow used a time vortex while he was in the realm and managed to escape through that. However, this could be untrue. After all, we have seen Ant-Man escape the Quantum Realm in the first movie as well and he managed to do that just by growing big. Perhaps the answer really could be that simple?
Time and space do not mean anything in the Quantum Realm, according to Dr Pym. However vague that is, that basically spells out everything we know about this so far. So maybe we will learn a bit more about how Ant-Man travels within the Quantum Realm and comes out. Maybe he finds some sort of a wormhole kind of a portal or maybe he uses a time vortex? We’ll have to wait till the movie releases to find out.
The biggest fear we have with regards to the Quantum Realm is that it might be treated as a huge ex-machina to solve all the conflicts in the film. It would be more interesting to have some rules set about the Quantum Realm, rather than an open space wherein anything is possible because…science.