The 9x4 probably won't work. It won't give the plane enough speed.
The 10x6 might work. It will make the motor work harder and will give you a little more power (although possibly a bit less speed), but the battery won't last as long (the motor might not either! [img]i/expressions/face-icon-small-shocked.gif[/img] It depends on what motor and battery you're using).

Find a place that has the right size of prop and buy at least half a dozen of them.

Also, make sure they're electric props, not glow engine ones (which tend to be larger and shaped differently, and also have a larger hole in the center). 10x6 is the most common size for glow engines.

APC and Master Airscrew have the prop you need (Tower Hobbies) GWS has electic props (Horizon Hobbies) Look them up on the internet. the 9x4 will work. Less pitch = climb prop. High pitch = speed prop. As oohex says it it important not to overload electrics. What ever the manufacturer recommends is usually arrived at after trials at the factory. Some adjustments in pitch and diameter may be made if your trial indicates the motor and battery are still running cool and performance is not degraded.