What is the ruling on a husband with four wives stating, 'Which of you have I not lain with?', if he has not specified a time limit?

Primary text

If a man with four wives states generally, "Which of you have I not lain with?" without setting a time limit, the waiting period for the divorce is tied to their lives. When any one of the wives dies, the remaining wives who were not lain with (as specified by the pronouncement) are each divorced by a single divorce. This occurs again when another wife dies. If the husband dies, all four wives are divorced at the final instant of his life.