"The estate is at the crossroads. The heir who wins the windfall will be the one who finds the FOURTH." That's it, that has to be it: The heir who wins the windfall will be the one who finds the fourth! (26.93-95)
While we read this text along with the heirs when Ed Plum discusses the terms of the will in chapter six, it's not until we reread them along with Turtle in chapter twenty-six that we all realize what's been missing all along. There never was any word at the end of the sentence; it's a trick hiding the real question that the heirs have to solve in order to win. And it's Turtle who figures it out.