I test drove S5 not S4 but same engine and transmission. (Only convertible has 3.0L supercharger with s-tronic. Coupe remains old 4.2L N/A with 6-speed auto)
From my perspective, performance wise, neither of them were impressed me at all.
Test driving both over 60k cars should be fun but it was kinda boring.
I was smiled once I left dealership with my 335is.