What mobo? If they'll play or can be helped to play together, most all of the newer ones have the basics of Intel's Flex mode incorporated, so you basically will have 8GB in dual channel and the odd 4GB from the 8GB stick running in single channel mode. Can check with CPU-Z in the memory tab it will probably indicate dual channel mode. This is assuming the 4GB and the 8GB will play, if you have any problems give a shout and we can try adjusttments