You need to enable memory remapping in your BIOS. In some BIOS's, you need to go to the advanced settings, sometimes via CTRL-F1. What motherboard do you have?
Note that you will want to make sure you have installed the vista-64 patch BEFORE changing your BIOS settings. I forget which one you are looking for but it's the one that prevents blue-screening when you have 4 GB or more of RAM in Vista-64.
Look again, that motherboard definitely has a function to enable memory remapping. Take a look in Advanced - Chipset or something. If you can't find it, search Google. Vista64 won't show all 4 GB until you enable this and it is definitely there.