what would you do if you have 20,000 miles on your tires and runover a metal piece that shreds one tire. Buy 4 new ones or just one and have the mileage difference between the 3 and new one?
That's absolute crap. Low air pressure on one tire doesn't effect anything on the 4wd and low air psi is enough to drop the diameter just as much as less tread. Hell most trucks I see have peeled one tire more than the other three. Never seen a 4wd breakdown due to a tire with a little less tread.If you do not replace all 4 then you will have to remove the bad one and bring it down off the truck.
For liability reasons there isn't a tire shop that should be replacing a tire on a 4X4 or AWD that is worn more then the spec they go by.
All 4 tires have to be within a certain amount of wear.