Sorry, the A6 and the Passat are the stable mates. I've got friends who do/did own each and a quick peek underneath will confirm their relationship, as would a look at the spec sheets (notice how the Passat/A6 motor are longitunially mounted whereas the Jetta/A4s are sidewinders). The A4 chassis is also shared with the Audi TT and various other European models in the broader VW family.

The A8/Pheaton relationship is a not quite as close as the A8 uses a unique aluminum frame vs. the Pheaton's steel frame. They do share a lot of drive train, suspension and other chassis pieces however. Even the Bentley Continental shares a lot of the Pheaton's basic mechanicals, albeit very modified.

As for the Stang/S4 matchup, sounds about right to me. This is a very apples and oranges comparison -- low cost 2+2 RWD sports coupe vs mid range 4 seat AWD near luxury sedan. And the Stang does spot the S4 some 400-500 pounds, which would readily compensate against the S4's higher HP rating, at least in a simple straight line speed contest. Make things a bit more challenging, i.e., throw in some tight bumpy curves on an unfamiliar road with a touch of rain and the AWD S4 might suddenly look like the preferred ride.

But in the end, both of these very diffrerent cars accomplish their mission and goals admirably and either one would be a boon to hage sitting in one's driveway.