At the end of Ant-Man and the Wasp, Scott Lang is left stranded in the Quantum Realm while conducting an experiment trying to collect more quantum energy. It perfectly set up the use of the Quantum Realm to travel back in time and stop Thanos from ever realizing his plan. How this will happen, we may have a good idea.
During Infinity War, it is revealed Hawkeye left the Avengers and took a house arrest plea after Captain America: Civil War to protect his family. However, seeing as he’s supposed to play a major role in Avengers 4 and the likelihood that his family turned into dust, he will be ready for some payback.
Last we saw Thanos, he was contemplating his accomplishment on some remote planet. We have no idea where this planet is, though some have theorized it to be either within the Soul Stone or on Gamora’s home planet. As for Captain Marvel, she is in some distant part of space from what we know. When her standalone movie debuts in March, we should learn more about her whereabouts. What we do know is that she was paged by Nick Fury and will be along shortly.
Article source: https://www.technobuffalo.com/2018/09/03/avengers-4-where-are-the-avengers/