Few things first...I'm assuming you mean 4GB not 4MB. 😉
You are going to have to replace the RAM in the computer. You can't just add to it. They put in 2 1GB sticks, so, you have to replace them with 2 2GB sticks. As far as where to buy, I want to know also. Prices are a lot lower than if you buy from Apple, that's for sure.