Who Murdered the Heiress?

What the count's eldest daughter wants is to get to the end of a ball alive. Her father has no sons, and to keep the estate in the family he has settled on four men of suitable rank for her to marry, a duke, a marquis, a count and a prince. The ball is where she is to name one of them as her first dance partner. Before she can, she is poisoned and dies on the spot. Then she is alive again, and she knows the poisoner is one of the four. She has ten returns in which to find him. If she uses them all up, her soul is gone for good. She goes back into the ball to look, and dies there a second time. Each return puts her at the same ball, with the same four men.