When you say only 2GB of the 4GB is showing, do you mean that System Information only says 2GB is "available", or are you saying that the BIOS POST screen only shows 2GB installed? If it's the latter, you probably want to replace the RAM. If it's the former, that just means Windows is using ~2GB for all of its services & applications. Download & run CPU-Z, & you should see that you have 4GB installed.