its never a good idea to mix two different RAMs. Also, it could possibly be a bad RAM slot, put the 8GB in the slot you currently have the 4GB installed in and see if the computer reads the correct amount of RAM. You could also try booting from one stick of RAM in the slot in question and see if it boots. If it doesnt boot, you may have a bad RAM slot. Conversely, some motherboards have particular requirements for which slots must have RAM in them, so be sure to check your motherboard manual.