Man what? Are we talking a Lego model city or something?
You have 4x40' radius blasts, so 4x80' diameters. That's not enough to level a city BLOCK, let alone a city. Even if we look at, say, London in the 1400s, you couldn't do more than seriously damage a single cathedral, or the palace of Westminster with that. Even bronze-age cities are far too big for that spell to do more than horrify people by blowing up the Acropolis or something.
Sure, there might be fire, but I guarantee a few well-placed lower-level spells could spread much wider and nastier fires than MS, especially with Control Weather or the like to encourage winds (to spread the fire) and prevent rain.