Replace the question mark with a number

A gentleman is preparing to board a plane, when he hears that the Pope is on the same flight. “This is exciting,” thinks the gentleman. “Perhaps I'll be able to see him in person.” Imagine his surprise when the Pope sits down in the seat next to him. Shortly after take-off, the Pope begins a crossword puzzle. Almost immediately, the Pope turns to the gentleman and says, “Excuse me, but do you know a four letter word referring to a woman that ends in ‘unt?’” Only one word leaps to mind. “My goodness,” thinks the gentleman, “I can't tell the Pope that. There must be another word.” The gentleman thinks for quite a while, and then it hits him. Turning to the Pope, the gentleman says, “I think the word you're looking for is ‘aunt.’” “Of course,” says the Pope. “Do you have an eraser?”