It depends on a lot of factors, but most likely, you wouldn't see any difference in performance if you did and were able to get them to run at 800 MHz. If you combined them, the memory will run at the slowest speed of the chips anyway.
Now, if you have a reason to need 8 GB over 4 GB, you'll see a difference with the increased memory, but only if the 4 GB is holding you back at this point.