So you can build a completely legal multiple-cascade system up to 32:1! The problem is that it would have only a fraction of the range of adjustment that you need. That's why the maximum practical purchase is 10:1, with a single cascade and two double blocks.
I am not familiar with the 4.7 rig, so you might want to try a 12:1 or a 16:1 double-cascade system, but I still suspect you'd block-to-block it before you reach your maximum tight/loose settings.
One way to lighten the load is to change to bigger blocks - 30 mm is the maximum allowed.
I sail in East long Island and it can get windy, plus I am small for the 4.7. How would the 12:1 be set up, and what blocks would be needed? Right now I my setup is two double blocks that are than connected to one primary line, that is attached to the vang tang, while the other control line is lead to the deck base to the cleat. Thanks guys.
PS no matter how much line is in the cockpit from the controls, it will be no where close to the amount of hardware in the cockpit of the 470! There are controls for everything from the rig tension, and jib halyard tension, to the jib cars, all lead back to the skipper.
You'd have to go for a 2 x 2 x 3 system. That would use three single blocks (one with a becket) instead of the two doubles on your current setup. There would be a primary line just like before, then a similar secondary line for another 2:1, and finally a third line that does a 3:1 and runs down to the deck and back to the cleat.
As I said before, I am skeptical whether a double-cascade system gives enough range of adjustment, even with a 4.7 sail that probably stretches less than the bigger ones. You'll have to test it. Be sure to mount the block on the mast as low as possible (tie it with a rope around the mast below the vang tang), and make the first two ropes short enough that the floating blocks and the cringle (nearly) touch one another when the system is completely loose. Even if it won't work it'll be a good simple exercise in building control systems, which applies to all sailboats.