My car will start every time I go to start it.

How long are you running the radio, a few minutes or many hours? Obviously the battery is running down. If it's four years old, it's nearing the end of its expected life and there won't be much "reserve" capacity left. Pretty soon it won't be strong enough to crank the engine. Also look for corrosion around the battery terminals. That is a sign the battery is going to fail in the next six months or so.