Speaking from experience, one 7 1/2 shotgun pellet from fifty yards away hitting me in the back of the leg, through Levi's, REALLY GOT MY ATTENTION!
This was on a quail hunt years ago down in Baja. I've also killed a coyote with one shot, with an ounce and a quarter of sixes at forty yards. all of this makes me respect any shotgun with any load. I think an ounce of #9's inside a house would stop anyone. And if it didn't the second shot at half the distance would for sure. #4 buck? I reserve that strictly for sea-going mammals that steal fish.