4 miles in 20 hours for PJs?!
That is some serious terrain.
As I under stand it each PJ had an over 100 pound pack and was pulling a sled with something like 400 pounds more. And they were traveling uphill to the crash site.
In my experience most people massively overestimate how fast they can cover ground. Given a moderate pack, less than one-third of their weight, and flat ground two miles an hour is as good as most people can manage over six hours without straining, hurting themselves, or tiring themselves to the point that performance suffers the next day.
Out of shape people, the majority of the US population, will need to go slower. People who are seriously fit will go a bit faster.
Two miles as hour is what I estimate as a good working number for the pace of a general evacuation on foot. This assuming moderate loads on healthy people, or little to no load if you include out-of-shape adults, children and elderly, and flat, easy terrain. Rule of thumb is that if you couldn't comfortable walk the path in cheap sandals your going to have to figure a slightly slower pace.