Your motherboard is dual channel and there are two ram slots. To make your ram operate as dual channel you need a stick in each slot.
If you did what you are suggesting and put a 4gb stick in one slot and an 8gb in the other slot, then only 4gb in each slot would operate as dual channel and the other 4gb wou.d be single channel.
That being said, there is no guarantee the sticks will work together. To give you the best chance of getting the sticks to work together you should make sure the ram is same make, same speed and same size.
Therefore you should buy another 4gb to match your current stick or get 2 sticks of 8gb and sell the 4gb stick.