I'm debating about putting the painted trim on the outside of the car onto my car ... im not 100% sure yet. I just want to double check that no modification is needed

 

There shouldn't be any modification. You will have to use your S4 pillars. And if you're doing the painting then you should use Vinyl paint from a good parts store.

 

i found that krylon fusion paint works quite well also

 

the trim on the fenders from an S5 have plastic tabs that lock into place in holes on the S5 fenders as compared to the flexible trim on the S4 which is held in place by adhesive. you would have to cut off the tab from the S5 trim and use some type of glue to bond to the S4 fender, and as for the doors i cant remember how they were held on the S5's. its probably the same way as the fenders. its been a few years since ive messed with the trim hope this this is some kind of help for you.