I just put a PR4 on the rear of my Bandit last week.
I was not happy at all with the PR3s on my bike. The front lasted only 6,500 miles (vs 10,000 miles for PR2), with no real noticeable difference in grip, wet or dry. The rear PR3 did a little better, lasting to 8,000 miles (again, the PR2 went to 10K.)
The PR4 is supposed to have the same grip as the PR3's, but longer mileage. Now, this is a standard (non-GT) PR4 that I just mounted, but the tread is noticeably thicker than on the PR3, and there is a slightly different design with the sipes. I can't give you any more info than that, because the roads are still so full of salt I don't want to ride again until a good rain washes everything off the streets.
So, I'm currently running a PR2 in the front (the PR4 wasn't out yet), and the PR4 in the rear. Very stable; time will tell on the mileage.
FWIW