Dire ses quatre vérités (à quelqu'un)
[deer say kahtr(eu) vay ree tay
to tell (someone) a few home truths, give (someone) a piece of one's mind, not pull any punches (with someone)
to tell (someone) his / her four truths
The French expression dire ses quatre vérités
means to be perfectly blunt and honest, telling another person negative things that are potentially hurtful.
I don't know why there are "four" truths in the French expression, but the possessive adjective indicates that those truths belong to the listener (the person hearing the truths), as opposed to the speaker (the person telling the truths). Anne thinks that Jean is selfish and rude, so she's going to tell him those truths about himself. In contrast, the English equivalent "to give someone a piece of one's mind" is talking about the speaker's mind. It doesn't really make any difference, but I think it's an interesting distinction.
Si tu as vraiment marre de l'égoïsme et l'impolitesse de ton mari, tu dois lui dire ses quatre vérités.
If you are really sick of your husband's selfishness and rudeness, you need to give him a piece of your mind.
Mon meilleur ami vient de me dire mes quatre vérités.
My best friend just gave me a piece of his mind.