don't think nitro play's a factor in chosing the F plug for a 4 stroke.
I am flying a Saito 65 5% Nitro and the OS F plug is doing fine. My brother is Flying a Laser 150 with 1% Nitro (or 0%) and also an OS F and also runs very well. And you americans always use way too much nitro and they also run nice on the OS F.
Maybe if he doesn't you can try another Four Stroke plug brand. But don't put a 2 stroke plug into an 4 stroke engine, cause most of the times it won't work