I really want a last blast >=6 cylinder petrol car

That may well be the case, but you also want comfort. Given that this desire for comfort is due to back problems, going for a sporty German car (as a general rule, these all have suspension which is too harsh for British roads) doesn't seem a very sensible choice.

As for your first post, yes, in theory you could put tyres with a taller sidewall on as long as the circumference of the tyre remains the same (within a small margin, can't remember for certain, but I think it is something like 2%). So you'd have to use a smaller diameter wheel to make the measurement the same. But, your insurers may not be too happy.

You’ll need to ensure the smaller rims fit over the larger brakes on the S4

Yes, of course, I was forgetting about that. Though I guess the brakes on an S4 won't be as big as an RS4.

Forgive me for my ignorance, but what are 'back issues' on cars? I've never heard the term applied to them before.

The OP has the back issues Andy, not the car!.

Ohhhh! How embassrassking! Probably a good idea then to go for an A4 as said. :-)