wheel spacers on S4?

Yes. go 8mm in the front and 12mm in the rear.

Use factory bolts and there won't be such problems as alignment, vibration and rattling noise? Thanks in advance for your response.

well, the factory bolts aren't going to do you any good. you will need extended ones. most places that will sell spacers will also sell bolts. when you put them on the car just make sure you use a wire brush to scrape anything extra off the surface of your hub and the spacer. then place them on and hit em with a rubber mallet to make sure there's no space. throw on your wheels then you are pretty much done. you shouldn't feel any vibrations or anything if you take the right steps. it won't don anything to your alignment either.

Thanks for the response! Much appreciated!!!

I'm trying to fix RS5 BBK to my S4 B8. But can't clear the 19' Rim. For the time being, maybe spacer is the only choice until I get a set of nice Rim. Anyone has this experience? How many mm spacer I should get? Thanks.