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.