Some motherboards are very sensitive to mismatched ram. It is possible that different batches of ram with the same part number can have a different manufacturing makeup. To be certain that this does not happen, ram is sold in kits.
That said, the possibility of a problem is not significant, particularly since you are ordering from the same place at the same time.
If price is not terribly important, you might look at a 8gb kit in a 2 x 4gb configuration. http://www.newegg.ca/Product/Product.aspx?Item=N82E16820231311
4gb stick prices have come down, and are about a 10% premium.
Such a kit will fill only two slots leaving room for expansion. Also, I have found it easier to overclock using two sticks vs, 4.