Have you already tried with the PAE (Physical Address Extension)? Actually 4GB shouldn't be a problem...
Anyway, try this:
sudo apt-get install linux-generic-pae linux-headers-generic-pae
sudo reboot now
I had the same problem, and after installing the linux-generic-pae and linux-headers-generic-pae my Ubuntu shows the whole 8GB I have available.
I hope it helps :-)

REF: http://askubuntu.com/questions/61221/why-is-my-system-showing-less-memory-ram-than-is-actually-installed

How To: ubuntu natty (11.04) on asus a53s (k53sv family)

Few configuration for Linux Use of a Asus K53SV Machine.