I even tried your settings above, I got the same hash rate for both -530 kH/s
I currently have many, many 4L's and every single one is above 600. I've gotten a few to 700 with almost no problems. It's all about the computer and cgminer.
What does the computer have to do with it? I've tried every setting I could possibly find on the net and then some - nothing. Certainly nowhere close to 700. The fastest hash I have seen is 650. If you have some settings then let this guy know so we can give them a try.
I say computer because you need enough memory, cooling, etc..
Second is the overclock. Straight out of the box you should be getting 550 with no overclock on I=19. If not something's up with your computer.