That's pretty cool. Does it actually stick to the S4? I saw that it is made for iPhone. Does it fit all the way around the S4's camera area and stick to the small metal rim around it?

As far as the magnet damaging it-- I very strongly doubt that the magnet will do any damage. I use a car dock mount that is pretty much just a round magnet. I put the thin round metal disc that comes with it under the battery door (under a cover/ case works, too) and that sticks to the magnetic mount. It's called iMagnet. I've used it with the S4 and also with my last phone, the GNex. No damage whatsoever, and I use it every time I'm in my vehicle.

iMagnet Mount ? Seemless magnetic mounting solution for smartphones and tablets

Post a picture using the lens converter when you get a chance. I'd like to see how it looks with the S4.
i still have the pre-loaded protective plastic on the camera's lens, which acts like a primer. So no i wouldn't know. I'm pretty sure it would that adhesive isn't one "not" to leave sticky residue