In short, yes - it is 4WD all the time.
In simple terms -
The car uses electric drive to power the front and rear axles.
When the battery is nice and full, power comes from the battery to all 4 wheels.
When the charge is 0, it is still in hybrid mode, but more of the time you will be in series hybrid, where the engine is generating electricity to still power all 4 wheels (delivered via the battery)
When you go above about 40mph, and definitely above 75mph(ish) then it will enter parallel hybrid mode if it deems it more efficient, where the engine is driving the front wheels and the electric motors are kicking in if required to provide extra traction/power. This is true even with a full battery. So technically there are times at those speeds when only 2 wheels (the front) are driven, which makes sense.
Though if you are doing more than 50mph in conditions that require 4x4 then you need to think again...
Just to clarify - the electric motors front and back mean that it is 4WD all the time.
The S-AWC system intelligently distributes power to the right wheels and is actually quite clever. It uses the ABS and other systems to control wheel spin. So, when in EV, series mode or even parallel with the rear motor on you have a cleverly controlled 4WD car.
"4WD LOCK" mimics a centre diff lock, which can be handy in particularly slippy situations, such as very slippy muddy fields/tracks and ice/snow, to ensure power is distributed evenly between front and rear axles, just like centre diff locks always have. It is not for 'turning on 4WD' ;-)
