Though everybody that's ever owned a 4WD has forgotten at least once in their life to disengage the 4WD so don't freak out if you do it. The biggest issue is you could damage gearing in the transfer box or differentials. If you're not hearing any grinding or whining noises that weren't there before, thank your lucky stars and try to never do it again.


But since he'll only ever be using 4WD on "slip" surfaces (dirt, snow, mud, sand, etc), it makes no difference if all the tires are the same wear height or not. Also, a gear based differential doesn't really care as different spin rates is exactly what it was designed to allow. Though a limited slip rear diff is common on lots of trucks so he may want to keep tires on the same axle equal to protect the clutches.


AWD doesn't have a transfer case that locks the front and rear axles together, hence they can drive on dry pavement. Most use a clutch based system, and any clutch is a wear part. Therefore, you want to limit the amount of wear it gets, which means minimizing how much it is forced to slip. Ergo, evenly matched tires all around. Toyota AWD systems use something similar to a torque converter to allow for slipping front to rear, so they aren't susceptible to the uneven tire issue.


As far as new tires front or rear, I've seen the test videos. I've also driven in real life in an southern east coast city with routine monsoon style rain. I now avoid the situation as I can afford to do so, but I've had plenty of experience with bald fronts and bald rears. I had way more close calls with bald fronts than I ever had with bald rears. Plus, in a truck I can get a lot more miles out of those worn tires on the rear than on the front. But that's me. I won't deny that the test track data says the opposite. I just know what my real world experience was.