TMZ has been sitting on this video for 4 years.  *WARNING OFFENSIVE LANGUAGE*

The video was shot backstage at a promotional event early in his career ... where Justin cracked this terrible joke ... "Why are black people afraid of chain saws?" His punchline, repeatedly saying, "Run N****r."

TMZ also reports:

The video was posted Sunday on a British pay site.  Sources connected with Bieber say he is "frustrated and sad" and now wants to address it publicly.

Is this just a case of tragically bad parenting or is something really, really wrong with Justin Bieber?