My notebook havin following configuration.

OS: Linux Mint Xfce 13

RAM: 8gb

Available RAM: 4gb (it should be 8gb or near by that)

Follow below instruction and you will be able to use full memory.

Enabling PAE manually

The PAE enabled kernel can be installed using the Synaptic Package Manager (accessible from the System menu under Administration -> Synaptic Package Manager): The relevant packages are called "linux-generic-pae" and "linux-headers-generic-pae" and should be easily found with a search for "pae".

Alternatively they can be installed using either apt-get or aptitude through the terminal:

sudo apt-get install linux-generic-pae linux-headers-generic-pae

After a reboot the PAE kernel should be booted as the default option in the GRUB boot menu.

To confirm that PAE was enabled correctly you can use the system monitor form the System menu under Administration -> System Monitor. The first tab shows usable memory.

Removing non-PAE kernels

If PAE was successfully enabled you may want to remove the meta-packages for the normal kernel, in order to disable unnecessary updates to the non-PAE kernels:

sudo apt-get remove linux-generic linux-image-generic linux-headers-generic

If you want to remove all of the old kernels from your boot menu, it seems like you have to do it all manually:

sudo apt-get remove linux-image-<version number>-generic linux-headers-<version number>-generic

Something went wrong

If something went wrong, just remove the PAE kernel packages using either synaptic or apt-get/aptitude. If you did not specifically remove the non-PAE kernel packages, they should still be present on the system and automatically selected as the default when the PAE kernel is removed.

The relevant PAE kernel packages are:

linux-image-generic-pae  linux-image-<version number>-generic-pae linux-headers-generic-pae linux-headers-<version number>-generic-pae

Source:

https://help.ubuntu.com/community/EnablingPAE#Ubuntu_10.04_LTS_.28Lucid_Lynx.29_to_Ubuntu_12.04_LTS_.28Precise.29