Your chip and board are designed to run in dual channel mode. This only works if each channel has the same memory in terms of size, speed and timings. If you mix types in the slots then the board will run in single channel mode and cut the bandwidth of the memory bus in half.
Go with hilltopmonk's suggestion of 2x4GiB. This will give you a lower latency than adding 2x2GiB of the same timings as the existing sticks. However, if cost is really an issue go with 2x2GiB, but make sure the speed and timings are the same as the existing sticks.