In addition to the previous answers, AWD and 4WD systems have important technical differences. A 4WD system uses a gearbox to mechanically lock the front and rear driveshafts together, such that they always have to spin at the same speed. AWD systems use a viscous clutch, allowing the driveshafts to turn at different speeds when some tires lose grip, at the cost of some wear on that friction surface.
That's why replacing the tires evenly is important on AWD, but not necessarily 4WD: different diameter tires spin at different speeds forcing the driveshafts to turn at different speeds all the time, wearing out that clutch prematurely.
Edit for completeness: on a 4WD system with differently-sized tires, the wheels would still be forced to turn at the same speed which means the tires themselves would be where the slip and wear occurs. That's why 4WD systems aren't supposed to be used on paved roads: having that slip is no big deal on a loose surface, but doesn't work well when all four tires have grip. (This also applies when all four tires are the same size, when the differential is locked and you're making a turn. The wheel on the outside of the turn has to spin faster the inside wheel, but can't, and so the tires scrub instead.)