I'm not sure if there's a specific discussion threda deicated to it, but a search across the Motoring Discussion and Technical areas of the Back Room will probably help, using 'tyres' and 4WD or 4x4 as the search terms.

In addition, Honest John himself often states the reasons when people ask him on his weekly 'Agony column', and teh archives of which can be found via the News section of the website.

In general, and from memory, the reason for changing all four at the same time is to avoid one pair or single tyre having a sufficent disparity in tread depth, and thus in rolling circumfrence, as the 4WD system will notice the difference in rotational speed and think one or more of the tyres are slipping and keep altering the amount of power being delivered, which presumably isn't good for a smooth ride or probably for the transmission.

I'm sure others more in the know will be able to elaborate more and to give the maximum disparity in tread difference before such an issue becomes a problem.

If you have one larger tyre then the differential on that axle has to constantly work to make the difference in rolling radius, but in the case of 4x4's (Subarus being permanent full time all wheel drive) the centre differential also has to work constantly to accommodate the same issue.

Rear axle on a Subaru could well be a strong LSD so the axle diff doesn't really want to comply with permanently 'slipping' so in theory could transfer more of difference to the centre diff, which also won't benefit from working all the time.

Then the VSC system might sense the slower rotating wheel (fitted with the now larger tyre) and trigger stability correcting brake adjustments.

These are just a few thoughts off the top of my head, no doubt others will know just how realistic these issues and any resulting damage would be.

In all honesty Subaru is one of the more robust AWD systems, but even so i wouldn't be putting either of our AWD Japanese (one Forester) through any more constant strain of the transmission than necessary, and i do rotate the tyres to even wear out.

You run the risk of burning out your diffs if your tyres vary in circumference. As well as rotating (I thought it was front to rear same side?) get the tracking and four wheel alignment checked every couple of years to keep everything wearing evenly. Its the price you pay for 4 wheel drive.