So to clarify, you are using a 4GB stick with an 8GB stick?
If this is the case, then you are not running dual channel. Plain and simple. You are running two different sticks each in single channel.
To make things worse, they are not the same regardless of the supposed timings and speeds along with the fact that they are two different sizes. This will inevitably cause instability in your system whether it be tomorrow or 3 years from now, who knows when, but at some point your RAM will fail because of it, and it could end up taking out other components along with it.
This doesn't mean you go out and buy another 8GB stick to put together with your current one. even if you buy the same one from the same place, it was not packaged with another stick as a kit and therefore it is not guaranteed to work properly when run with another stick. At which point you take the risk that it won't work properly or will hurt your system.