In this video, where the 3rd chick is born, and rolls out of the nest, the parents are in a dilemma going off the nest with the remaining egg, and covering the chick, back and forth. I sent the link to Brad Semel, asking if that might be why the 4th egg failed, as I learned in my previous inquiry that the egg stopped developing after ~24 days.
He replied that "I have never observed such asynchrony in hatching. It was likely scalded and died in the absence of an adult providing thermal regulation. Yes, your video may have captured the cause. "