Mental and Physical Endurance Peeks in a 4 hour duration. Unlike the movies and in fiction, realistically even human will-power has limits and function in economies (according to Emotional Intelligence).
In very difficult tasks, Mental exhaustion is much a part of physical exhaustion because the reserve needed to push the body to its limits requires concentration and an effort of will.
Remembering the 4 hour rule comes in handy when planning for days resources. Knowing the average performance of everyone else, lets the players or the GM prepare according to the 4 hour rule.
Consider it a deadline, because after this point energy and concentration will have diminished significantly there is significant diminishing returns with certain continuous action.
Fatigue is more a Mental burden than a physical one, even if the body is rested. Pain, Will, Reciprocity, diet and contagious morale all affect the disposition of consciousness int he course of the day.
With the 4 hour rule, use it to prepare for optimum firefight activity for the opponents and not just the PCs. PCs basically try to use these hours more efficiently than their enemies, getting more done and gaining the most cost efficient strategic advantage in the course of the day.
In extreme discomfort or temperatures: cold or heat half this duration.
In airsoft, this means I pack as much gear as i would be able to use in 4 hours even if the event is supposed to be "whole day". If it is several days, I pack for 2x the 4 hour measure. The standard helps prevent over-packing or being under-equipped.
In your more realistic combat scenarios use this as mark for emerging mental fatigue that requires Sleep or its psychological equivalent.
The 4 hour rule works with most people. People with specially conditioned sleeping habits may ignore it. These are people who can reach a deep sleep in a few minutes and achieve what others would normally need 8 hours for mental replenishment. In GURPS this is less sleep advantage, meditation, deep sleep perk, etc. in real life, for normal people its part of soldier conditioning and sometimes found in efficiency geeks.
the use of sugar, caffeine, taurine (in sports drinks), nicotine, alchohol, over-the-counter medication regarding its affects to the observer and the people they know is an interesting source of information regarding performance efficiency.
We all have different metabolism, neurochemistry, and conditioning. there are drugs and conveniences like coffee that allow us to compensate for some pretty common disadvantageous dispositions. First hand experimentation in Min-maxing concentration and work performance by optimizing habits along with chemical augmentation is a very useful source of information about one's self and first hand awareness of mental limits and economies.
The level of self awareness and effort in personal management are natural outlet to apply Gamer Min-maxing and problem solving skills.