I am not sure this will cure the pitch down on rudder. A better solution would be to mix it out if you are using a computer radio. Consider most of the current crop of aerobatic airplanes....the CAP 232, Extra 300 series, Giles.....all have the stab on the upper fuselage line similar to your 4*40.

However, it would be interesting to see if it makes a difference. If you change it (1/2" suggested) let us know if it works!

Clair