Bismuth and Matrix are very similar to lead. With a full choke, you can kill ducks to 60-70 yds. with 4's. That said, I've never known anyone who could marry a full choke pattern with a moving bird at those distances with regularity, only luck. That's why I'd suggest sticking with the modified 5's if you're a good shot. Over decoys at 40-50 yards max, use 6's.