Though we know Marvel is working on Phase 4, the only movie we know that's been announced so far is Spider-Man: Far from Home. Interestingly enough, Marvel Studios is said to be going to Italy to shoot a scene set in WWI.
This comes from Badtaste.it (via WeGotThisCovered) who says that the production is going to Piedmont to shoot around a castly in Racconigi, Cuneo. Translated, the description of the scene says it's "a story set during World War I, centering around a huge attack on the city of Sarajevo."
With Marvel already said to be casting characters for The Eternals, I have a feeling this scene being shot is going to be for some kind of after-credits scene. Rumor has it that the movie would focus on the character Sersi, and it's possible we could be getting some kind of historical montage where we follow her in history.
Otherwise, my guess is just as good as anyone else's. We don't know how far off production has gone for other upcoming MCU films, but we do know that movies like Black Widow, The Eternals, Shag-Chi, Doctor Strange 2, and Guardians of the Galaxy Vol. 3 are in the works.
For now, Marvel Studios is gearing up on the release of another period piece, Captain Marvel. The time jump won't be so far though, with the film taking place in the 90s. After that, it's smooth sailing to the release of Avengers: Endgame.
Catch Captain Marvel in theaters March 8, 2019.
See Also: Captain Marvel Says Thanos Is A 'Shrimpy' Enemy