With 4 channel there's servo (recoil) movement with elevation. The work around is use an aux channel and mix that in with 125% elevation. What that does is tells the Tamiya unit to fire as if you pulled full stick without doing it and isolates the servo from te normal elevation command.

Sounds confusing, but it really isn't, the only draw back is you can't set the right stick like a HL, but opposite, left stick drive or Tamiya's left throttle/right movement.