it seems that a 4th age of Civ7 should be Mad Maxish.
But I am guessing they aren't going that route.
Well, IF there is a 4th age included and it is not the Medieval (I’ve come to some thoughts confortable with having it, but in these I would rename it to “Feudal” and have some mechanics related to managing a semi-independent empire for it to be trully unique) but a future age, it would be most likely DLC.
If it is DLC, it would be optional, and it being optional, they can go all the way with it and provide multiple, alternative ages with a different, semi-fantastic theme.
This is, they can try to extrapolate on current contemporary and try to introduce mechanics and victories based on a network-connected, information-war, globalised word.
Or go fully domsday to a hellish scenario (may be not fully red-death), but something mad max with civs fighting for limited resources.
They could go fully ecopunk and devise a world were fossil fuels are forbidden and the most sustainable civ gets the best benefits.
Or maybe trully ciberpunk with the world being covered in metallic cities, wit cyborgs and robots making warfare…
You may chose which future path you want at the start of your game, or make it random / depending on how 3rd age ends. Or just de-activate the DLC and don’t worry about any futuristic nonsense (wish I could have done so in VI wit the GDR…)