A Fourth Age? No way!
Perhaps against the grain on this, but a Fourth Age to me will only exacerbate the issues with Ages and the Modern Age rather than be a fulfilling addition. (Plus, with like 200-250 turns proceeding Modern, I don’t think we really need an extra 100 turns to finish a game, imo.)
I like the Ages system. But it’s not without its downsides. Sometimes, the transitions are minor changes. Sometimes, the crises are mild inconveniences. Often times, you don’t get to feel the rewards of the later Age buildings and units.
I think with the Modern Age, the right call will be to extend the Age (progress points to reduce chance of Score Victory) but to also add to the Victory Projects while adding to the Tech and Civic trees to aid in pushing towards Victory (and less on new units and buildings you’ll probably never use).
I think this could help make Modern Age Civs also feel a bit more balanced. Those Civs that are geared towards Victory are very strong while the rest seem kinda just there. Some Wonders are quite late that their bonuses seem unimportant. Extending this out with new Victory Project add-ons (and Legacy Path changes) as well as longer trees might help them not feel like side bonuses on your way to Victory.
There’s more to discuss like scaling bonuses per Age (Like, are you going to pick an 18 Gold per turn Endeavor when you make 500 GPT?) and how production and gold and urban sprawl are handled.
If a Fourth Age comes, I hope it’s down the line, towards the end of the game cycle (like Game Modes in 6). Deeper Ages, for me, are more preferable than more Ages.