You need to find out whether you have 32 bit or 64 bit windows 7 installed. 32 bit Operating systems are limited to under 4 GB of RAM. You can put more in but it will not recognize nor use it. Go Control Panel > System. It will tell you there.
My first inclination is to assume you have a 32 bit Windows 7, but that can be wrong. There are other things that can cause your problem if you have a 64 bit OS installed. So to get things narrowed down please tell us whether you have 32 bit or 64 bit.
I just got 8 more GB's of RAM for my Desktop, but it gives me the good ole "3.99GB usable out of 8GB" in my computer properties, and as someone who hosts Game servers this is just annoying. I have looked around the internet consulting the great Google oracle, but I haven't found anything really helpful. So if someone here could give me some more direct help that'd be awesome!
So to clarify: How can I have my Computer access all 8GB of the installed RAM instead of just 3.99GB?
OS: Windows 7 Home Premium
RAM: PNY Optima 4x2 pack
MOBO: GA-P55-USB3 (Gigabyte)
CPU: Intel i5 core (760 @ 2.80GHz 2.79GHz)