Arzak

Will the older D3 part number 4E0601025AT wheel even fit the D4 properly?
I know the offset is different but I don't understand what you mean by the smaller center bore?

JZK26

So we almost discussed this to death, and I was so close to pulling the trigger but then it hit me. Question: I currently run the OEM 21" in my 2017 S8. To the best of my knowledge, these are not forged. If that's true, would getting a forged 21" wheel with all season be a good solution for year round? I really love the look of 21" on my car.

MP4.2+6.0

Hmmm. The center bore change is a basic years break point form "older" Audi model year wheels to "newer" ones, so be sure to know the issue if you are looking at used OE wheels. Audi increased the size of the center hole from 57+mm to 66+mm between the D3 and D4. Started with the A5 IIRC, was phased into the A4, came out with C7, was always there on Q5's. Only the "smaller" Audis with transverse motors akin to VW's kept the older 57mm size. The bolt pattern remained the same, but not that center hole.

The import is if you put a newer wheel on an older Audi, you can use a simple + cheap hub centric adapter ring--though not my personal preference. if you try to go the other way and fit an older wheel on a newer Audi, you have to use a combination spacer and adapter only sold by a few companies. Those retrofit adapters make the offset more aggressive (if not desired...like for winter sets ups). It is not really practical to machine the wheels to increase the bore size, and the few who try it outside a professional machine shop tend to mess them up.

MP4.2+6.0

Correct, there are no forged OE 21's. The S8 factory wheels cost 9 pounds of unsprung weight per corner on my S8 before I liposuctioned them in favor of factory forged 20's. Relative to practicalities of (a little more) sidewall for winter potholes and all, even less difficult a choice for me to go to 20's. Yes, I suppose you could find a forged 21 aftermarket, but likely pretty big bucks. Often a value subtract too as cars age in the used market, where "stock" (OE) is an easier sell.