It is true that in some applications, your choice of motor (petrol or diesel) may provide distinct advantages, however there are benefits for both in 4WD vehicles.
Reliability, simplicity, low down torque and better fuel consumption of the diesel engine in long haul outback driving is unquestionable. Coupled with the fact that diesel fuels are readily accessible in remote areas, a diesel engine is the logical choice.
However the flexibility, smoothness, instant power and broader rev range make the petrol engine better for general driving and especially good for escaping mud and sand traps as well as lunging up steep irregular hill
climbs, where opportunities for traction may be limited, but the instant power of the petrol engine allows maximum momentum to be gained in a very short distance. Factors such as lower initial cost and faster warm up times, coupled with a higher power-to-weight ratio also means that petrol engines are better suited to shorter stop-start driving conditions.
With newer and more reliable technology giving both types of engine more power, economy and better overall flexibility, you will be able to make any engine more versatile and able to perform better under extreme circumstances. These factors somewhat reduce the arguable difference between the petrol and diesel engines used in 4WD vehicles.
For a more detailed discussion on this topic see our article Why Diesels are Different