How we cite our quotes: (Part.Letter.Paragraph)
Quote #4
I was distressed, I shall admit, at the unfavourable opinion you seem to have of [Valmont]. I detect your mother's prejudices there: it was to conform to them that I so long neglected this truly amiable man […] who, after all is trying to reunite us when your mother has separated us. (2.72.4)
Danceny implies that Cécile's negative view of Valmont is colored by her mother's prejudices—a likely scenario for a mother trying to protect her daughter from dangerous men of their acquaintance. Valmont and the Marquise, knowing this, do their best to alienate Cécile from her mother and substitute their own influences. The novel suggests that young people are particularly impressionable and are easily convinced of what you want them to believe.
Quote #5
My first object was to secure a reputation for being invincible. (2.81.34)
Notice the language here: the Marquise wants not so much to be invincible, as to have the reputation of being invincible. If you have the reputation, that determines how you're treated. If people see you a certain way, then it doesn't really matter if you're not really like that.
Quote #6
I shall now write one to Madame de Volanges, who is sure to read it out in company, whereupon you will hear the story again as adapted for publication. (2.85.33)
Madame de Merteuil has just told Valmont how she got Prévan arrested for a crime he didn't commit. She'll be telling the tale again, but in a heavily revised version. Because she's sure Madame de Volanges will pass the letter around, she'll be able to deceive the public. They'll assume the story they hear is the truth, because the Marquise has spent years creating the false reality that she's trustworthy. Valmont assumes the story he heard is the real deal. Is the account he read any more reliable than what the general public will get?