I don't understand your post at all.
You list 8GB.
You say you only have 4GB.
You say you want to ad another 4GB.
So which is it? You either have 8GB plugged in or 4GB, not both.
How do you have 8GB showing as 4GB in single channel? That does not make any sense at all. I apologize if it sounds insulting but I keep re-reading your post and I just don't understand.
I can only guess that you have a 32 bit system that limits your ram to 4GB (see link):
http://msdn.microsoft.com/en-us/library/windows/desktop/aa366778%28v=vs.85%29.aspx
If that is the case then no matter how much ram you add you will only be able to use 4GB. If I am correct about your OS being 32bit (x86), then simply upgrade your OS to 64bit (x64) and you will see your additional ram (Your 1 8GB stick will be fully utilized rather than limited to only 4GB like it most likely is now) and the confusion written above will all be clear.
Starting here, when you add another 4GB, you will have a total of 12GB. How off am I from understanding you properly? I do want to help
While you would now have 12GB total, dual channel systems usually have the same of everything ie 2x6GB for 12GB with same brand, make model, cas latency, voltage, and speed will be a true dual channel. Your 1 x 8GB + 1 x 4GB for a total of 12GB will not necessarily run in dual channel as there are discrepancies at least in size and perhaps other areas and at best run in asymmetrical mode which is not true dual channel. In effect you will just have more ram and are likely not to gain the benefits of dual channel. I would recommend getting the same second stick of 8GB instead if you truly want dual channel. Some will say you will never need so much ram but if you render, or are looking out for gaming in direct X 11.2 (see articles on tiled resources for direct X 11.2 to be released this autumn: http://www.bit-tech.net/news/gaming/2013/06/28/directx-11-2/) this then can be to your benefit down the road. Hope this helps! Have fun!
Thanks,
Justin S.
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