Is this something I can do? I know people say you aren't supposed to mix and match RAM, must install in pairs, etc etc. But I accidentally got thermal paste in one of my RAM slots and I can't fill all four. I want to upgrade my RAM a bit, and I don't need a full 16GB so can I just add one more 4GB stick of RAM into the last working slot? And would it HAVE to be the same exact RAM I have installed now?