All four points mentioned above combined to form "class" of link for that specific website, the term "class for specific website" is important to understand because a link might be benefical for some website and helps a lot in improving page rank and might be search engine position (depending upon how link is backed to the web page) but this doesn't necessary mean that it will be beneficial for every other kind / category of page. Every webpage has its own way to define the class for backlink, depending on four above mentioned factors.

Relevancy plays an important role to define class of link for different websites. If the back link is relevant then search engine will analyze the relevency and give credit to linked website. On the other hand if link is from totally irrelevant website then it might penalize website depending on number of irrelevant back links.

The more relevant back links, the more it will boost the page rank of the website, but quantity is always effective with quality.