I have the 15.5x12 4 blade on my os140rx and I couldn't be happier. I get many compliments at the contests about how much time I have to set up for my next maneuver. My caller has more time. Downline speed is nice and slow. Upline has plenty of power. Nice and quiet. Also its in a 10 1/2 pound Zen 140. Not the 120 version, the custom built 140 version. Anyway, I recommend this to everyone and everyone I recommend it to that has tried it loves it. It really is the prop to use. I have found no strange thrust issues just needed more right thrust. I was running a 17x13 and I really liked that until I tried the 4 blader. Now I have several 17x13's sitting around that will not be used as I love the 4 blade.
Good luck
Chris