Trying to match that one RAM stick will be a crapshoot. You will need the exact same make and model for best results, even then the model may some differences. At least try to find memory which is the exact same timing (detailed like 9-9-9-24), voltage and speed for the best chances of dual channel performance.
If you want a better shot at dual channel performance. Sell the single 4GB and buy a 2x4GB matched pair. Then all that needs to happen is for the two separate matched pairs to get set to the same timing and speed. Which should be done by the motherboard.
If you want absolute guaranteed results. Sell all the memory and buy a 2x8GB or 4x4GB matched set.