Sudden Crash with sudden yaw

Hi all,
Recently we were testing our newly assembled 650 mm quadcopter with 14-inch propellers. The motor used is JFRC-U2820 420kv. With a take-off weight of 3 kgs, we were getting a flight time of approximately 40 mins at 40% throttle for hover. After testing it in hover conditions for over 2 hours in multiple flights, we were testing it in forward flight and also trying to move it around but suddenly we lost control of it. The quad started to yaw and flip and fell from 60 feet in height. It took multiple flips and multiple 360 yaws before crashing.
We are wondering what could have gone wrong and the reason for such a catastrophic experience.
We did, however, the motors and ESCs after the crash and there doesn’t seem to be any problem with them.

I request the experienced group members to leave their comments on the problem.
We are new to the world of pixhawk and are excited to learn more about it.

I am attaching the flight log for your reference.

Loss of thrust on Motor 3 (Command to max), attempt stabilization by dropping command on opposite arm Motor 4, overall loss of thrust with rapid Return To Earth.

What FW version?
Doesn’t 4.0.x prevent this from happening (provided you have enough thrust reserve using the remaining n-2 Motors) ?

He’s using 4.0.3 in this log. And after switching to Stabilize and applying full throttle it still went down.

Added throttle input.

I’m not sure about an update in 4.0.4 for this issue. But perhaps I missed that.

Probably under-powered…

I meant 4.0.x, but yes, he should update to 4.0.4

His thrust/weight is good I think. MOT_THST_HOVER at 0.26 and:

Quite in contrary to your assumption, it did hover at 40% throttle in full charge