Mixing non-identical memory, even if the timings appear the same, is a crapshoot. It used to be a non-issue but as DDR has gotten faster and faster, tolerances have all but disappeared.
You can get lucky with it but if you have any plans for overclocking, forget it. Overclocking and mixed memory are almost a certain fail.
True. when i mixed it was back in the day. Right now i have 2 x 2gb sticks, 2 x 4gb sticks. Same Brand, voltage, timing and everything.
I would just save and get a 8gb kit, or buy 2 4gb kits.