Egypt (and possibly Persia depending on what we get) would probably want to be retooled if the 4th age is ancestral.
Also, I struggle to imagine a 4th age being as engaging/mechanically complex. Not that I would mind a "hunter-gatherer" style mini-game, but I'm not sure if it could flesh itself out enough to be justified as a full era.
EDIT: Although, the "menagerie" building certainly makes me wonder if we could be getting an animal collecting mini-game. Gotta catch 'em all! (and then, inevitably, GOTTA PET 'EM ALL!)
I don't want to go too much in-depth, because this is really Off Topic, but for complexity, there are numerous Technologies that actually date back to the Neolithic/Chalcolithic, such as:
Agriculture (9800 - 4800 BCE)
Animal Husbandry/Domestication (9000 - 4400 BCE)
Carpentry (Mortise and tenon joints - 5000 BCE)
Calendar (8000 BCE - calendrical circles)
Metal Working (Copper - 7500 BCE, Lead, Silver, Gold - 6500 BCE, cast copper - 4700 BCE)
Pottery (6000 BCE - low-temperature kilns, 4200 BCE - fast potter's wheel)
Sailing (coastal - 5500 BCE)
Weaving (4500 BCE - warp-weight looms)
Irrigation (5000 BCE - first irrigation canals)
Fishing (7000 BCE - nets, 6000 BCE - whaling with harpoons)
Archery (6000 BCE - self bows)
Masonry (9000 BCE - walls and towers)
That certainly gives a basis for a bit of 'complexity' in a Prehistoric Age - and, of course, would also require reworking the Antiquity Tech Tree as well.
Nor is the period entirely without substantial settlements: Jericho and other Middle Eastern 'city-oids' from 9000 BCE to 4000 BCE, numerous Chinese settlements with central ceremonial structures, walls, etc from 5500 BCE, other city-like concentrations in parts of Europe (a whole previously-unsuspected bunch of Bronze Age stone fortresses in the Balkans, for instance), and cities with apparent central planning and smaller versions of 'henge' calendrical circles in what is now the Sahara.
Biggest problem I can foresee is that we have no historical Leaders of any kind for anybody that far back, Answer, I think, would be to use Mythical Leaders from their descendant Civs: Herakles or someone resembling him shows up in legends from Celts as well as Greeks, so with work a sampling of 'Leaders' could even be compiled.
Assuming anyone thinks that work would be worth it for the game.