#4 is the bold that mounts the bracket the caliper mounts to. It has a normal head. #8 is the allen headed bolt that holds he caliper to the bracket. Which bolt is it that you are trying to tighten? You can find a replacement for #4 at almost any hardware store, as it's a normal metric bolt. #8 can be gotten at a BMW dealer.
If you take the bolt out, look for damaged threads or metal pieces on it and where it came from. I have seen where #8 was installed a bit crosswise and needed to be removed and correctly threaded.
Hope this helps,
vince
Originally Posted by pgrindstaff