I made homemade yogurt 3-4 weeks ago that still looks and smells good. Can I reheat homemade yogurt to make cheese?
Depends what kind of cheese you want to make.
Yogurt cheese is made just by straining yogurt. Your yogurt would not need to be reheated and, as long as it is still good, you can strain it just fine.
Other cheese varieties either use acid or acid and rennet to tangle up milk proteins. Although some home recipes will use yogurt starters to achieve the necessary acidity, fully fermented yogurt is too acidic to remake into another type of curd. The milk proteins are already denatured and coagulated into the yogurt gel.