4wd works through a transfer case at the tailshaft of the transmission. usually the gears for the front wheels in a 4wd system move faster then the rear to allow for more wheel speed during turns.
AWD is a front wheel drive system that usually has some type of iol pump/torque converter in therear differential which turbs the rear wheels when the driveshaft is moving, say, 50 rpm faster then the fronts. when the fronts lose traction, the driveshaft spins faster then the rear wheels are comparetively. so the torque converter kicks in and starts putting power to the rear. thus AWD.. a good diagram of this is on TV the next time you see a SUBARU commercial, right when thay say "the wheels that slip to the wheels that grip" check out that little diagram they have going. also check out the subaru forum on this site if there is one.