Just crashed my newly built quad, from logs it appears clear to me that i lost motor 4, for whatever reason. I someone else can diagnostic something more specific all comments are welcome.
Crash happened in loiter about 2 meters off the ground, it just looke like motor 4 stopped (it looks like that from logs). Motor 4 amd esc 4 both work.
The only thing we can come up with is thermal on the esc because it is closed in a plastic motor holder with no air circulation, but reading the esc manual it looks like it doesn’t have any thermal protection so it should have just burn.
Here is my configuration:
Quad 6 kg
Motors Foxtech X5010
Esc Hobbywing Xrotor 40 Amp
20x5.5 props
PH 2.1 3.6. RC10 Chibi OS
Custom built frame
2x10600 Batteies 6S
If i forgot something important on my configuration please ask.
Log: https://drive.google.com/open?id=1JuPEl3RXHSScXAzCY24fCHmiUTDLvq_l
Corrado
you are right with motor 4, and it did not stop completely.
thats a plus on the “thermal throttling theory”.
from the xrotor website “Low-voltage cutoff protection, thermal protection, and throttle signal loss protection all make the ESC more reliable” 
escs need airflow, does not matter witch one. there are always losses and with no airflow at all it will fail. mater of time. i would improve the airflow, and make some tests strapped on the ground for 3 min full throttle.
Thanks, Quad is already under surgery to solve the esc thermal problem, even if it is not the cause we better change it anyway.
From the Hobbywing site this is what they say about thermal protection under the FAQ:
“This ESC is specially designed for multi-rotors. It doesn’t have the LVC protection, thermal protection, but the start-up protection, overload protection and throttle signal loss protection.”
Even if it is a bit criptic i understand it doesn’t have LVC and Thermal protection.
From what you can tell the motor didn’t stop completely?
Another thing that maybe contributed is that i may have done a mistake when i made the throttle calibration and have changed timing from intermediate to high, but there is no way to know on wich setting the esc is now.