If you try to use a 15x4 it will probably not be able to turn enough rpms to keep the engine in the power band (15600 rpm). Try a 13x4, if that works OK then you might want to try a 14x4. All the forumulas will only provide a guideline so that you do not have to buy every possible prop size when you experiment.

Trying a 15x4 should not hurt anything... as long as you respect what your ears and eyes tell you when the plane is in flight. If it sounds like it is bogging down, or looks like it is barely flying, get it down and let it cool. Then change to a smaller diameter prop.

The formulas project 17.5 pounds of thrust with a 15x4 at 11400
16 pounds of thrust with a 14x4 at 12500
and 14 pounds of thrust with a 13x4 at 13800.