Im pretty sure you can't mix 2 and 4 GB Sticks. Other than that its important that both run on the same frequency.
Not entirely sure if you can mix various brands, but i am pretty sure yours will not work (2+4gb)
Contrary to what the others say, mixing ram should work, but then again it might not. To be on the safe side, use identical sticks, as recommended by your manual. But theoretically it should work with two different pairs of identical sticks, possibly even with four different sticks but that's just asking for trouble.
Note that all sticks must work with the same specs (speed and voltage). I think the Kingston HyperX requires 1.65V or runs only at 1333MHz in 1.5V, which means you might have to run everything in 1333MHz.
If you want to try it, to benefit from dual channel you must install each pair in the same slot color. Assuming the first two sticks are correctly installed in the blue slots, you just need to install the new sticks in the two remaining slots (white).