Just a quick comparison in case anyone else is thinking of an RS4. FWIW I have the B7 Avant, it's a 4.2L V8, no turbos but they can be supercharged to 600+bhp.
First of all, it's not easy to compare the Supra and RS4 as they're not like for like. They're both fast, but in different ways. In a nutshell, the Supra is aggressive, noisy and exciting - the RS4 is more refined, modern and smooth.
As I'm getting on a bit now (50 next year) and have a young family the RS4 ticks more boxes. It's also better for the bumpy country roads that are round where I live, the Supra is better for motorways, dual carriageways and fast roads.
I much prefer the sound of the V8, I always thought the sound of the Supra was just 'noise' which I didn't really care for.
RS4 power delivery is a straight line, yes it's fast but not in an exciting way like the Supra and its turbos.
RS4 handling is way better, it has 4WD and bumps in the road are hardly noticeable. I can also put stuff in the boot and kids in the back.
RS4 doesn't look all that, let's face it, it's a clumpy estate car albeit not as bad as a Volvo. Supra looks awesome and you're sat low down with that wraparound dash.
As far as mods are concerned there's not a lot you can do with an RS4, I'm getting a Stage 2 map next week and a carbon clean which should bring 430-450bhp. After that it's a massive leap for a supercharger (about £12k). So the Supra wins hands down for mods both engine and cosmetic.