As I mentioned before, a 15x4 will probably put the engine RPM in the right range, but it will not make a suitable prop for the airframe. Insist on 8-9" of pitch. A 15x4 at 9000RPM will give a pitch speed of 36MPH. Stall speed of your model could be 20-30MPH so you'll be flying around at full throttle just to stay in the air. It'll probably crash on take off. You want a pitch speed of over 60MPH minimum, preferably over 70.