I agree with everyone. You're most likely experiencing a restriction from a 32-bit OS. The most RAM that a 32-bit OS will recognize is 4GB and that includes the memory on your video card too. That explains why you aren't even seeing the full 4GB (because of the video card).
Ex_bubblehea mentioned something about mismatched RAM. I'm actually unfamiliar with this phenomena. I know back in 1995 you needed to do some voodoo with matching ram modules, but anymore, memory is pretty flexible. Would you mind explaining how that restriction works? I'm curious.