MCU: 16 Things We Learned From 4Chan's Latest Marvel Movies "Leak"
11. Hank Pym And Janet Van Dyne Will Appear In Infinity War
The Rumour
The supporting cast also includes Ant-Man's veteran pair Hank Pym (Michael Douglas) and Janet Van Dyne (whose expected casting for Ant-Man & The Wasp is still unconfirmed).
Response
Having the pair appear is a little odd, since you'd have to expect that any superfluous figures who didn't need to be written in would simply be ignored, but if they have a good reason to be involved, then so be it.
Obviously, this rumour makes absolutely no sense, unless it only refers to Avengers 4. In that case, it spoils that Ant-Man & The Wasp will end with the return of Janet Van Dyne from the Quantum Realm, as foreshadowed in the first Ant-Man. Perhaps that dimension hopping knowledge will be why Pym and Van Dyne are involved.
That would presumably mean that Hope Van Dyne would be involved as well, though it's not explicitly said.