Your machine (I have the same one) comes as 2 x 2GB RAM modules.
It will work with a 2GB stick and a 4GB stick (for 6GB total), but as noted above, memory speed will be optimized with two matched modules. Depending on what you do, it may or may not make a noticeable difference.
But since the price of an 8GB kit (2x4GB) is only around $74, it's reasonable to just go the full 8GB. Sell/trade the original memory on eBay to make back a few bucks if you wish.