I understand that Adder, my post is just giving the most common method that you would inject number 4. I know that it's not a sterile method, but when done properly is fine. The biggest issue is clean needles, sterile cotton and clean water. Ideally you would have:
Clean needles (always the same), distilled water, Isopropyl for cleaning injection area, micron filters, pharmacutical grade heroin (Diamorph), a RN or phlebotomist to register the shot, and a sharps container to dispose of the used works.
I realize that the above is not really practical and that's why I made the post that I made. No, it is not the 'safest' method, but it's a lot safer than some methods I have seen, and that was the goal of my post, harm reduction, not harm prevention. Real harm prevention would be flushing the smack down the toilet.