The issue with TS4 worlds is due to a very odd design decision. From what I understand, the game renders all of the objects on every lot in a given neighborhood -- stuff that can't be accessed or used unless the player goes through a loading screen. NPC Sims can't use the objects either. Loading all of that eats up a lot of processing power.
As a result, the developers have a genuine dilemma on their hands. They're expected to provide beautiful, evocative worlds. However, they can't provide too many functional lots. If they did, players would fill them with huge, elaborately-designed buildings crammed with stuff ('cause that's what players do)! The solution: Fake it till you make it. That is why Snowy Escape ended up with a ghost town neighborhood with nothing in it but set dressing and magical, Tardis-like vending machines instead of shops.
If you look at TS4 worlds chronologically, they're clearly becoming more like movie sets with each subsequent pack and it's only going to get worse.
