Once that happens, we’ll probably see a lot of things shift into high gear, and that could include Stan Lee filming cameos.
We know he films cameos for several films all at once, so it only takes a couple of days for Lee to become a major part of Phase 4.
Having said that, the MCU’s Phase 4 is probably further along than we think.
While only the sequel to Spider-Man: Homecoming and Guardians of the Galaxy Vol. 3 have been officially announced, and only one of them has a release date, Marvel has said they don’t want to take away from the conclusion of Phase 3, and thus won’t be officially announcing much else until Avengers 4 is over.