You could do 2x4GB or 4x4GB, for a total of 8GB and 16GB, respectively.
Question is, do you have anything that can saturate 16GB? Yes? No? If no, will it saturate 8GB? Yes? No? If yes, get 16GB... Better to have too much than not enough.
I got one of
these
That exact kit I can't find on Newegg, however. Only the 1333Mhz, 1866MHz and above variants.