We will need more details on your system's specs, what you've stated is generically laid out specs. Download and install
CPU-Z and pass on a screenshot of each tab in the app, upload the images on imgur and then paste the links to them on this thread.
Did you buy the system brand new? I assume not.
Think of ram upgrades like a pair of shoes, if one out of a pair got lost, would the shoe salesman sell you one out of a pair or would you need to buy a pair from him? No matter how hard you explain to him, he's bound to sell you a pair. Same goes for the ram slots and the ram. When you have a kit this makes less work on your system to negotiate with the memory and ensures a stable and smoothly operating system. This brings me back to buying a kit of wither a 4x4GB DDR2 kit or a 4x2GB DDR2 kit with all of them being of the same make and model obviously going to happen since they are as a kit and not individual sticks.