You can but overtime they use different manufacturing components. That is why people buy their ram in kits. Motherboards can be sensitive to ram with different manufacturing components but there is no way of knowing for sure. It may work for you upgrading with a second 4gb stick later on or you may have problems. That is why if you're even considering 8gb (which you should), I'd say you get them 8gb now and save the possible hassle later.