Hi there , as far as I know you can you can go with different frequency but same capacity . Your entire mem will be running at the lower frequency , so with 2x2gh 1600mhz and 2x2gb at 1333 you will be running at the lower 1333 MHz .
Best answer selected by ami_subi.
@pshemax: you are spot on
I am able to run all my memories but at the reduced speed of the slower RAM (1333 MHz).