Well normally what happens, assuming the RAM is compatible, is you will have 8 gb's of DDR3, but at the reduced speed of the lowest set. I don't recommend mixing RAM types/speeds up, but I'm sure it's been done before. I would rather buy 8 gb's of DDR3 and sell the single set of 4 gb's of RAM to offset the cost, or keep the extra RAM set around for a bit until you feel your new set has no issues (sorta like a back up set).