You might have something like a Series Landrover transfer box where in 4WD mode the front and rear axles are locked together and in 2WD mode only the rear axle is driven.
But does that make a Defender, RRC, or Disco with LT230 transfer box which has a lockable diff AWD or 4WD? The centre diff supplies power to both front and rear all the time, but can be locked in slippy conditions otherwise if one wheel spins *none* of the wheels get drive - the axle and centre diff will both spin.
Then what about the later Borg-Warner transfer box in later Range Rovers, where you still have a centre diff and epicyclic gearbox to set high and low range, but there's no diff lock - there's a massive viscous coupling that turns it into a ridiculously stiff LSD.
But in modern times, technology has most definitely blurred the distinction. Higher-end Jeeps also have a “full-time 4WD” that sounds an awful lot like an AWD system that can also do 4WD stuff.
So, eh, it’s probably not a good idea to argue too much about broad categories anymore. Individual systems must be compared