Hardly any point in fitting a tyre worn to 4mm. It's 1mm off being Donald anyway.
I'm afraid I, and even the best tyre monkeys, are not blessed with X-Ray vision, or prescient powers. If people can't afford to run a car properly and keep it safely maintained then it shouldn't be on the road. Fitting second hand safety equipment is a daft idea.
Far to many people buy cars that are beyond their means to maintain and run properly - the proverbial champagne car on a coca cola budget. If less people worried about their egos and public displays of brand name willy wagging, they could drive a more sensible car instead and properly maintain it because parts will be a fraction of the price, and other running costs will be cheaper. We've all met someone who scrimped and saved for an M5 [insert alternative name of fast/expensive car here] who then had to put remoulds on it because they were living beyond their motoring means.