Only 3326MB RAM intead of 4GB in Vista 64-...

Hi,

I'm experiencing a little problem here with my Vista Ultimate 64-bit. Both the little gadget in the sidebar as the Task Manager tell me I only have 3326MB RAM available, while I have 2x Aeneon 2GB DDR2-800 CL5 installed in my system, in ganged mode. System information as well as the BIOS though, tell me I DO have 4GB.
I am running an MSI K9A2 CF-F, which has NO memory remap option in the BIOS, and an AMD Phenom X4 9550.

So here's the question: does my PC utilize the full 4GB's of RAM I have installed, or doesn't it. And if it doesn't, is there any way to make it do that?

Thanks already,
Yannick.