David,
You can read this thread https://discuss.ardupilot.org/t/heliquad-rudder-4th-servo-not-mixing-with-the-collective-acting-as-tail-rudder/43255/96
where I and some other users provide input. Also you can read this thread https://discuss.ardupilot.org/t/input-please-2019-recommendations-for-pixhawk-pixhack-for-450-sized-electric-heli/45495
where some advice is given. I don’t have experience with the Pixhawk 4 mini so I can’t comment. You can see my comments on the other threads.