the 4gb limit includes graphics aswell, so if you have a 1gb graphics card be it either onboard or via a card, you drop yourself down to 3gb even if you have 4gb pysically in the machine.

iv seen boxes with 256mb onboard with 4gb pysically in them only show 2 / 2.25 usable. usually this is down the the hardware not utilising the full 4gb allowed