[My5ABaby]

Bare minimum to go from S5 to S4 belts you would have to change the A pillar, B pillars, headliner, and obviously the actual belts. I'm not sure if any additional holes will need to be drilled.

To make it look good you would also have to swap in S4 rear carpet and bins as the S5 bottom B-pillar plastic is chunkier. Also the center armrest is different, but interchangeable between series. To make the seatbelt swap look right I would also swap the console. The S5 center console is basically useless.

As for other electronics, the S5 and S4 idiot clusters are not interchangable without some repinning/rewiring. About 1/2 are different so the wrong things will light up. That said, if you want to go from early S4 to late S4 or S5 dash (different idiot cluster design) while keeping the S4 under dash wiring, all you have to do is flip the plastic cover upside down so the top is sticking out farther, rather than the bottom. Side note: also to go from an S4 to S5 dash you have to add a ground to the S5 dash. I can't recall where, all I know is there's a spot on the S4 dash and not S5.

The plugs for the lighting by the rear view mirror are different between series, although the wiring seems to be the same so it would be an easy swap. The sunroof wiring is the same.

The window switches are interchangeable.