"Four men live in the city."
Translation:Quattuor viri in urbe habitant.
"In urbem" = in + accusative.
Is used in the Duo sentence "De ponte in urbem descendimus." for instance.
It means you have a move. See, the verb is "to descend", it's "to descend into the city". There's a motion verb.
Here, it's static, they "are" in the city. There's no in+ accusative.
It's an ablative. In + ablative means it's something in a static position.
Like "to be", "to live", to sleep" etc. No motion outside or inside the city here.