if your S4 calipers are in perfect nick, its unlikely to make a huge difference.
Most S4 calipers are, at this stage, a bit knackered however. The S4 caliper is also huge, limiting your wheel choices, and has extremely limited pad availability, with many manufacturers not producing pads for them.
The ATE calipers i mention above solve all of these issues. they're newer, more slimline, less complicated, and have significantly better pad availability.
They're a minor upgrade or improvement to the standard braking system, rather than a huge difference you might get fitting some 365mm 8 pot brembos from an RS4.