I would venture a guess that on steep grades and very low speeds like they were driving they would be better served in four low to keep engine RPM up. This keeps the hydraulic pump in the transmission spinning faster and pushing more fluid.
From owners manual:
4-low
This setting engages the front axle and delivers extra torque. Choose 4 low when driving off-road in deep sand, deep mud, or deep snow, and while climbing or descending steep hills. While driving in 4 low, keep vehicle speed below 72 km/h (45 mph).