Does your motherboard only have 2 slots? And you already have 2 2GB modules? Your post was quite unclear.
You can get 4GB modules but they are very expensive.
I am wondering why you are upgrading your RAM from 4GB, for most users 4GB is all that is required. What are you doing with your computer to warrent this upgrade? Or are you just looking to upgrade something, if so this is not the thing to upgrade.