The memory will only run as fast as the slowest memory in there if the two are different speeds. That is if they will even work together at all in 4x dual channel. You really up the odds when going 4x and buying separate 2x kits, even of the exact same model, of them not working together nicely (blue screens, random errors, etc.).
There's a reason memory is sold in 2x and 4x kits by vendors: they are tested at the factory to work together. Memory made at different times can have microscopic silicon variances that causes one module to "hiccup" with another. I've noticed this issue more frequently since DDR3 was introduced.