What's the best way to lose as much weight as possible in just under four weeks?

Low carb, low fat: Fasting and missing meals is not a good idea. It just promotes Insulin secretion, which is not what you want if you're trying to lose weight. A low carbohydrate, low fat diet is effective. Eat vegetables, low fat dairy products, and protein, but avoid sugars, starches, and alcohol. You probably won't want to live on that diet, but it's an effective way to lose weight.