I built two Model IVs and while building the first one there was talk on the Kitfox List about the possibility of failure of the torque tube. As I recall the discussion started after a guy had his fail on short final. Lots of discussion on the welds and hardening of the metal as Dave suggests, but I seem to remember that most of the stones were cast at Mig rather than Tig. Most guys were adding gussets to the four pedal tubes and the rudder arms. I did it on the first one during the build - 1993 kit and the original owner did it on his 1992 kit before I took over for my second build.
Edit: I talked to my best Kitfox Bud last evening and asked him about the rudder pedal torque tube and he has never done the mod. He has owned several Kitfoxes including four Model IVs. So I guess it is us belt and suspenders guys that did the mods