[Adrian E]

There are always exceptions to any rule but generally new tyres replaced in pairs should go on the back. From a work perspective the reasons are mainly:

If you get a tyre failure it's more likely to be on an aged tyre so if that's on the front you will get under steer rather than oversteer in the event of a blowout, so less likely to be catastrophic in terms of accident risk ( doesn't help if you get a nail in a rear tyre obviously!)

Rear tyres generally do less work so a brand new tyre can bed in gently which will increase its total lifespan

Tyres moved off the rear will probably have more tread but also be older - moving them to the front then allows them to be worked harder on steering duties and with greater load on them. You can usually feel the difference when rotating tyres as the sidewall will feel firmer initially

Quattro does mean you're ideally looking at rotating tyres (ideally diagonally) once and then replacing all 4 - I've done my last set in pairs as 1 pair were worn out on the shoulders and the others still had 4mm+ so I ran them for 7 months ( only about 2k miles) and have now rotated the ones I fitted in September to the front. Tread depth should be fine assuming you're running matched pairs of tyres or if there's no appreciable difference in rolling radius (you'd be surprised how different it can be on nominally same size tyres!) - the owners handbook says the difference between rolling radius on used vs new tyres will not damage the drivetrain