How interesting, thanks for posting these. I'm not sure, though, that the ones of Princess Grace quite 'capture' her.
I thought the same, though I may be influenced by the lack of color compared to the other three and I thought the same about the portrait of Prince Pierre. That being said, I'm much more used to seeing younger portraits of him rather than when he was in his more senior years. Of course, Princess Grace was one of those figures who could capture more in a formal photograph than in a great many painted portraits by the most skilled painters around.