It's because you have 32 bit Windows. It doesn't matter what operating system it is, Linux and Mac have the same issue. It's based on the architecture built behind the 32 bit standard. It physically can only comprehend 3.25-ish GB of RAM. You can pump your system with 16gb of RAM and it'll still only acknowledge 3.25-ish.
64 bit will acknowledge all of your RAM. 64 bit has its limitations too, but I think it's substantially higher than what any personal computer would be at for a while.
EDIT - didn't realize you said you were on 64 bit already. Oops.