Weird crash on calm day

Hello I was wondering if anyone could help in diagnosing what caused a drone to crash. The quadcopter was running arducopter 3.6.5 on a Pixhawk 2 Cube Black that was purchased before 2019 I believe drone was built in august of 2018. The user didn’t listen to update to arducopter 3.6.8, but i don’t believe that had anything to do with the crash. It was a hot day in the 90 degree Fahrenheit and the drone was flying perhaps at 3mph at 50ft. As it was doing its lawn mower path the drone had a weird hiccup, recovered then right after lost control and “death spiraled” to the ground. I don’t have a ton of experience analyzing data flash logs but it looks to in my eyes that motor 4 was having issues providing thrust during the hiccups. But i was wondering if anyone else could look and provide their insight. A link to the log >

Your assumption is correct.
As you can see motor 4 had a hiccup, recovered, then failed

There was a curious current spike during the hiccup but not during the failure.
Bad ESC? Motor winding?

Classic mechanical/electrical failure.

I don’t think it was the motor but perhaps the ESC? In your experience do ESC’s fail intermittently? Because the autopilot was swapped and test flown and flew well. But perhaps the ESC’s need to be swapped to avoid any future mishaps… Thanks for the help.