alt Hacker News4WD: Distribution of power to the wheels is static. AWD: Distribution is dynamic.
4WD vehicles are often part-time 4WD, meaning they can be switched between 4WD and RWD or FWD.
AWD vehicles are full-time AWD because part-time AWD is unnecessary given that the drivetrain delivers power where it's most needed.
There are plenty of AWDs that operate as FWD 90% of the time, only engaging the rear as needed. (I am told this is for fuel efficiency but the logic escapes me.) But I have yet to ever see a 4WD that could be "switched" to FWD.
The clearer distinction is that a 4WD has a second gearbox (aka a transfer case) that either manually or automatically engages the front wheels, the Jeep Wrangler /TJ being the architype. That means the extra wheels cannot be triggered seamlessly. A 4FW has distinct driving modes whereas an AWD uses clutches to bounce between modes as needed.
Bluey has a couple lines about this distinction. In Australia, even small children have opinions on 4WD v AWD.