Get the OEM A4 Kit.

the paint cracking issue that a lot of people experience.

Plus I think the s4 sides look, looks more subtle and is less showy.
I don't plan to get full bodykit, just paint the stock front and rear and get the s4 sides. Will this look good? I don't know how low the s4 sides will be compared to stock. I don't want the sides to be lower than the front.

But I guess if you only want a subtle upgrade, then its an option. Otherwise you could just paint your lowers. It seems like "Heaps" of the US boys do this S4 sides option.

If you do go for the OEM A4 skirts, you must do the rear valance and from lip, else it looks rude.

PS. I painted my rubbers 6 months ago, and have yet to have any cracks whatsoever. They used heaps of rubber compound in the paint. I am also careful when I open the doors to ensure you do clip anything.

Personally, If I bought an S4, I would consider uninstalling the S4 Sides, and putting the A4 OEM skirts on.