It is possible. There is a somewhat increased risk of incompatibility, and you'd need to confirm with your motherboard manual how it treats that configuration with regard to dual channel/flex mode.
I would perhaps suggest you try adding just a second 4GB stick. That ought to be enough for now. And to maximise your chances when it comes to compatibility, try and aim for memory running at the same voltage, and ideally at the same clocks and timings too.