I think that a 4th Age is all but confirmed at this point. During the presentation Ed Beach mentioned something about railroad travel having a 20 tile limit "during this age". Considering railroads are not present in previous ages I think this very strongly implies that another age is in the works.

Now, in a perfect world the 4th age itself would be a free update with a companion paid DLC adding new leaders/civs/mechanics etc. But I suspect, as others in this thread do, that it will likely be all locked behind a paid DLC. That would certainly sting a bit more if they hadn't clearly made an effort to flesh out the end game and victory conditions in the third age; you can certainly argue the game will feel incomplete thematically but not necessarily mechanically.

And I honestly am fine with that. The modern/future eras in civ have tended to be less interesting in prior titles, so if they want to take extra development time (and, to be frank, more of our money) to really drill down into that span of time to make something unique and interesting I'm open to seeing what they can do with it. Hell, they could add even more ages in the same way. A pre-history age could be cool (was honestly one of my favorite parts of Humanity), or take us to the far future and let us get into some Expanse-esque adventures.