Its not a very good idea to mix and match memory. Memory should always be installed in matched sets all at the same time. In theory it will work but you might have stability issues. Assuming you have a dual channel CPU and Motherboard you would either want 2 * 4GB matched set for 8GB or 4 * 4GB matched quad set, or 2 * 8GB matched set.
Also if you have 3 out of 4 slots populated you will not be able to take advantage of dual channel memory settup - again assuming you have a dual channel memory controller on your CPU/Motherboard combination.