What prop are you using now, you don't say?

You won't hurt the engine with an 11x4, though it might not be enough prop for the engine to turn in the best portion of the torque curve. Basically, the engine would turn up to a higher than necessary RPM. You'll get a lot of noise, and the plane won't fly as well as it might with some other props.

If you put way too small of a prop on an engine, it will, in fact overheat. Basically, the engine tries to over-rev, which leads to too much heat. But you won't get those effects with an OS FX until you really get up there in RPM. www.osengines.com lists the max RPM at 17k, but I suspect you won't hurt anything until you excede 20k (haven't actually tried it with this engine though) (then again, OS engines "used to" have a liner peeling problem, so keeping the RPMS down might be more important on an OS than some other makes).

The 12x4 and the 12.25x3.75 look like the best bets, as others have said.

Btw, sometimes you actually go a bit faster by dropping a bit of pitch. The reason is that engine RPM doesn't remain constant, a lower pitch at the same diameter will allow the engine to turn up to a higher RPM, which may lead to a higher top speed. So you drop the pitch and increase diameter in part to keep the RPM about right. And the extra diameter helps all by itself, as noted above.