Supermarket's don't produce anything. They buy stuff from distributors or producers and then resell it. Are you suggesting supermarkets you go to are buying things for x and then selling it for 4*x, or are you suggesting that they buy it for x and then sell for (x+c)*4 where c is a proportional amount of all their other costs?

I'm talking about profit margins which subtract out all their costs. Everything I can find says that grocery stores typically operate on 1%-3% profit margins. Nowhere near the absurd rate you're suggesting.