At the risk of over complicating this there are two definitions of endurance that apply to light aircraft
Flight for endurance means that power setting which will result in the longest time in the air and will always be the minmium amount of power required to sustain level flight for the conditions of that flight.
The other is what the word implies, how long will the aircraft fly with a specified amount of fuel which is 3.9 hrs in this example
Fuel required is the fuel that has to be in the aircraft to complete the flight taking into account all factors, start, taxi, runup, climb, cruise, descent, approach and landign plus reserves plus a suitable extra amount for contingencies.