There's no real story progression in 4, NPCs basically just walk around town then grow old & die and new ones generate. There is a story progression mod out there, I think it's MCCC, you might check the mod section of these forums, they can help you out of you want it. The max limit is your played or liked households, but I think it does affect the number my NPCs your game generates.
I tried going back to Sims 3 for a while & still have an active save right now, but I couldn't get the Nraas mods to work & to me it's just not the same without them.