How we cite our quotes: (Part.Letter.Paragraph)
Quote #4
Old ladies must never be crossed: in their hands lie the reputations of the young ones. (2.51.1)
Madame de Merteuil has the sense to know that her reputation is not entirely in her power. She has to pretend to abide by the established social expectations that everyone else does. For one, don't be indecently late to parties. That won't do.
Quote #5
Were I to grant you as many talents as we [women] possess, how far should we still surpass you in their exercise by reason of the continual necessity we are under of putting them to use. (2.81.12)
The Marquise is explaining to Valmont here why women can be so powerful in seduction. Men are, by virtue of being born men, granted power over women. Women on the other hand, granted no power by society, acquire their superior seduction skills because they have no power—they have to constantly practice pretending to be coy and submissive. Men use their inborn authority to control; women have to use cunning.
Quote #6
At my entrance to society, I was still a girl, condemned by my status to silence and inaction. (2.81.24)
Even thought the Marquise was born wealthy, and "entered" society (like wealthy debutantes are still "presented" today), she knew that she was born into a class of people who were seen as inferior: women. Their opinions weren't sought out or valued, and they had nothing to do. Silence and inaction. Her determined self-creation was a reaction to this powerlessness. Notice that all the female aristocrats are known by their husband's titles.