From previews of Ant Man we know that the driving plot will be the Microverse/Quantum Realm and that the OG Wasp played by Michelle Pfeiffer will be rescued. It was slipped in an old interview that she will be present in the large battle in Avengers 4 as well. They will deal with the threat in the movie and then the end will cross over into the ending of Infinity War where Hope Pym will get snapped. Scott or Hank will approach Tony and they will begin working on a way to use the Quantum Ream for time travel.
Avengers 4 will probably take place 5 years after the events of Infinity War where some of the avengers travel back in time to the battle of New York to warn themselves or simply to infiltrate SHIELD in order to kidnap Loki for information on Thanos to collect, destroy, hide, or use the stones so that when they go back to their timeline all snapped individuals are still present.