For SC4 i have not really tried to reach a max. My best City was at 1,000,000 while using the exact quarter of a big city.
My Record for SC3K is 3,155,000 People!
Without Cheats!
With maximum wealth!
With Maximum Land Value!
End even with normal Streets and Stuff!
So i can not hold on with the guy above, but i cared much more about realism. So its another approch. And rich buildings are looking much nicer. Here are some Pics:
Here is my standard Pattern for SC3K, its nearly unbeatable, rich residents are popping up like mushrooms.