can I make tomato sauce with just 4 tomatoes?

I think you can do that. Do a recipe search to get some inspiration. There are raw sauces and cooked sauces. If they are really good tomatoes, a raw sauce might be great. Many cooked sauces call for tomato paste which serves many functions but can also help you stretch them.

Sure. It will just be a small amount of sauce depending on the size of the tomatoes.