I was going to guess that his motherboard has an on board graphics system that borrows memory directly from the ram which is 'on board'.
The actual value of memory that the on board graphics chips uses can depend on the settings in the bios set-up. When windows boots up it reports (in the 'My Computer" properties that you have less ram in use for the system than the total hardware installed.
You can run a program that is quite successful in detecting what kinds of hardware's you have like your actual total ram (maybe you got ripped off?). There is a free one that I like called:
Lavalys Everest Home Edition
In your case it could be that the on board is using somewhere around 512mb or some number. Thus removing an average of around 600ish...4000-600 = 3.4 and actually is a close number.
Good Luck!
