This is a common issue and I have come across of this question numerous times on the ASUS Forums, so I have decided to post some info regarding this issue.
If your system has 4GB of RAM or have upgraded to 4GB (Or more) and running a 32-Bit Operating System, It will most likely say you have around 3GB of RAM. Unfortunately there is no way you are going to see all the RAM you paid for.
On the other hand, if you are running a 64-Bit Operating System (Which is designed to support 4GB more), you might be able to see all the RAM but it depends on your motherboard Chipset and the BIOS need to support memory remapping feature.
If you really need to have 4GB or more RAM, you need to check whether the motherboard chipset support at least 8 GB of address space and it must support 64-Bit architecture.
Furthermore, you need to run a 64-Bit Operating System such as Windows XP 64-Bit or Vista 64-Bit to utilise all the installed RAM.
I would recommend you read through the following URL Here.