There is no "supposed to be"
The amount of memory shown is entirely dependent on the amount your system uses with other components, it depends on video card memory size and all of memory using devices in your pc added together.
I've seen pc's report 3.25GB.
If you wish to test the integrity of your memory modules them run Memtest86+, the latest version is 2.11, download the .iso file, burn it to a cd, reboot to the bios, set your optical drive to first boot device, hit F10 to save the changes and off you go. Let it run for at least an hour, (preferably 3 or 4 hours for best results, I let it run overnight to be sure) if you have faulty modules you'll see the results appear as red on the blue screen.