Could be battery or alternator. Go to Wal-Mart, and buy a Schumacher battery charger in the auto section (buy the middle one, I think it's 12A for $50). The beauty of it is that it can be a trickle charger, battery charger, as well as diagnose bad batteries. It tells you how much of a charge you have left (%), and what voltage is coming out. It works great, I used it on my Viper after I left the key in the ignition and ran the battery down. Also diagnosed wife's Cayenne after I wired heat seaters to the wrong hot wire and ran the battery down (battery was bad, and charger told me that).
4 years is probably a battery change scenario, but I'd buy the Walmart charger anyhow for future use. Who knows, your battery just might need to be topped off.