How many dimm banks does your mobo have? If it has 4 for example there should be two slots with matching colors. By adding two identical sticks in matching slots you enable the dual channel memory functionality.
My guess would be, that since the sticks have different capacity/speed they are not working together.
You could try to run the 4gb stick alone and if it's working, installing each stick in different colored dimm slot.
Also check the speed and timings of the sticks, you might have to underclock the faster one if there is a difference. Match it with the slower stick.