I have a 4 year old daughter, here is what we started doing. We have colored plastic coins. Each thing she does or can do is worth so many coins. We also have a bank that the coins fit in. We have a chart, so we don't have to argue about how much the coins are worth. The chart has a picture of how many coins something costs, with a picture of each item. So for example, our daughter loves gum, so there is a picture of two coins that are next to a picture of a stick of gum. Candy costs a bit more and a toy costs more. She can clean up her toys for a coin, no arguing, she can be helpful to her little brother and may get a coin, if she helps with dinner or laundry she gets a coin. She doesn't have to do anything, but she knows that she doesn't get coins and then doesn't get prizes. If we go to the store and she doesn't have any coins, there is no discussion of wheter she gets a prize or not cause she knows.
Another bit of advice, you are the mom, you are the boss. Demands from children should not be met. It is your decision whether he gets a toy or not, who cares how much of a fit it causes. My daughter knows not to expect a toy just because we are at the store, she may be sad that I don't buy her one but she fully understands the rule.
Good luck!