Right after I discovered the bodies, and before collecting evidence that came to light later in the day, I did call the lab about a necropsy. They said the cost is $208. I decided to wait and see if I could put the pieces together and save the $208.
With help from everyone on this thread and elsewhere, I have come to the conclusion that this is what happened:
It was raining off and on at dusk and I was in my room watching Stranger Things with my earbuds in and the windows closed. So if there was noise outside, I wouldn't have heard it. Later, I had a Zoom call which went longer than I expected, so I wasn't able to close the pop door inside the hardware-cloth surrounded run until 10 pm. It was pitch dark and raining, so I released the rope holding the door up quickly and in the dark, trusting that all 4 were inside as they always go in on their own at 9:20 pm.
The next morning, I found the two white ones dead inside the run, next to their outside waterer, which was still upright on a brick. The two brown ones were inside the coop as expected. At first, I thought the chickens died from illness because we examined the corpses thoroughly and there were no wounds.
However, one of the chickens had a lot of feathers missing, which remained a mystery for most of the day. Examining the feathers showed no skin or blood on the point of the shaft.
Later, I noticed the unmistakable signs of a bear having been very near the coop; but didn't attempt to get in (no breaches, scrapes, scratches, or damage to the hardware cloth). This led me to believe that the chickens reacted to the presence of a bear near the run.
I think the bear came by, the girls quickly went up inside the coop and the two, what I know now were males!, stayed in the run to defend the females. But they panic'd and flew straight up, breaking their necks on the floor of the coop above them in that end of the run. This must have happened while I was watching the movie or on the Zoom call, because the girls came down from the coop after it happened and probably tried to 'wake up' the alpha chicken by pulling its feathers. Then they went back to their roost before 10 pm, when I went out and quickly closed the door without checking on them.
That's the best explanation for the evidence that I am aware of. Thanks everyone, for your input and helping me solve this mystery.