Hello Chris,
Allfoad = 4 wheel drive?
If so, then you raise a very sensible question, since miss matched tyres/tread depth can influence the efficiency and longer life of the 4 wheel drive system.
Having said that, I cannot see any problem with simply renewing the damaged tyre since its axle partner has only covered 4K miles. I would however replace with the matching make and pattern.
Kind regards,
Gareth.