There is a very subtle difference between **** and ***.
**** has a rounder, moister, more rascally sound to it suggesting a similar nature to the person there described. It can also be effectively used as an exclamation of disappointment.
*** is more nasal sounding, a more linear and a shorter sounding word suggesting a type A personality.
The short harsh sounds of the word "***" are, when the word is properly used, more reflective of a person who takes little pleasure in anything, while the more protracted sound of "****" suggests that the person is more of a sensualist.
Compare with the German word "Arschloch", which has similar connotations to "****".
The two words are very different, and should really be considered separately. Culturally speaking I dislike "***", but sometimes it is the correct word. In Dickens' Oliver Twist, for example, when Bumble (I think) is talking about the pedantic understanding the place of the wife by the Law, he says that "...the law is a ***—a idiot."
The concept of the "Idiot" is the concept of one of no redemption, a willfully stupid individual who knows no better and does not care to. In the Irish culture, the "Eejit" (also spelt "edjit") is considered to be the Hiberno-English equivalent or translation, used primarily in the local dialect of English, and subsequently subsumed into modern Gaelic. Similarly, it refers to someone who may have behaved in a stupid unknowing manner. However "Eejit" can be applied with an affection for someone, and to all intents and purposes it is not a willful foolishness.
In this, these words are similarly to "***" and "****".
Does this clarify?
EDIT: Ah, just seen the censoring. ****.