Actually I have no issue with "for her sake I think it's best" from a detachment perspective. The reason I don't like "for her sake I think it's best" is that the other party is likely to hear "I know what's best for D4".
You already stated in the previous sentence "as you know it's very important to her" -- that places the focus on D4's needs and not your opinions about what is best for her. Your H can choose to honor D4's wishes or not.
