The latest fuel for some Avengers 4 plot detail speculation arrives in the form of some new merchandising art, which reveals that at least a few characters in the film (Thor and Rocket Raccoon) will be getting new suits - suits which look conspicuously similar to the Quantum Realm exploration suit that Hank Pym dons in Ant-Man and the Wasp:
To be fair, these suits could have any number of functions in the context of Avengers 4's story; there are probably a fair number of fans who will immediately make the jump to a scenario where the team needs suits for something like deep space travel. However, the fact that the suits are being worn by Rocket and Thor in particular is somewhat telling: both are characters who have traveled to some of the deepest regions of space, and never had to wear any kind of space suit when doing so.
Going back to the theory that similarities in design between these Avengers 4 suits and Hank Pym's Quantum Realm suit: if Avengers 4 takes place years after Infinity War (as is rumored), then Hank Pym's research would definitely be one puzzle piece that minds like Tony Stark, Bruce Banner and Shuri would want to delve into and explore, as a possible way of changing The Snap. Ant-Man and the Wasp used its third act and post-credits scene to introduce a lot of heady ideas about what the Quantum Realm really is, as well as the possible applications of its unique energies and time vortexes, and that work could very have been to set up connections that will become important in Avengers 4. Hank Pym latest scientific entries would've included rescuing his wife Janet from the Quantum Realm after several decades, as well as questions about the nature of the quantum energy powers she returned with.
Similarly, Captain Marvel's first trailer has hinted that Carol Danvers' powers could be tied to Quantum engergy, would could make her the literal key to helping the Avengers embark on this course of action. That would tie both Ant-Man and the Wasp and Captain Marvel (the two Marvel Cinematic Universe movie between Avengers 3 and 4) to the far-out plot that's rumored for Avengers 4, which could see the original team (plus a few new friends) traversing space and time in order to erase Thanos' destructive act before it can occur.
Avengers: Infinity War is now available as a digital download and on Blu-ray and DVD. Upcoming Marvel Cinematic Universe movies include Captain Marvel on March 8, 2019, Avengers 4 on May 3, 2019, and Spider-Man: Far From Home on July 5, 2019.