the difference between all wheel drive--and 4 wheel drive

Both allow the vehicle to provide power to drive the car at all four wheels. Typically, 4 wheel drive is selective where the driver can select whether he wants to be in 2 or 4 wheel drive. All wheel drive keeps power going continually to all 4 wheels. No selection must be done for this to happen.

For best fuel economy you want to shut off 4WD and save on energy to run the transfer case & the differrantial needed to run the two extra axles.