Truly sorry for not elaborating, however I wasn't sure that what I did is that important.
I have flashed ArduPilot 3.6 release candidate 7 (ArduPilot don't have a release yet for Pixhawk 4) , configured everything, calibrated and tried to fly. I had the same issue exactly as with PX4. Posted the logs on ArduPilot forum, exactly as I did here, however got responses from two members.
- One member that reviewed my log noticed and said that I might be too cautious and that big quads often do a little flip like movement, and that I might want to push or pull the pitch stick just a tiny bit before takeoff to force the FC to balance that flip like.
- second member offered me to set the MOT_PWM_MIN to 1000 and MOT_PWM_MAX to 2000.
The next afternoon I had a wonderful flight (though landing was a bit hard due to my poor flying skills).
In retrospect, I believe it would be the same with PX4, however ArduPilot does have Auto Tune which I wanted to use - for my no flying skills and no knowledge of tuning. Big quads does seem harder to tune and crashing would be very expansive .
However, my conclusion from the little experience I have with the Pixhawk 4, lots of grief for the novice flyer/builder.