I think 9 x 10 x 10 x 10 For the unit digit: 0-9 = 10 For the tens digit: 0-9 = 10 For the hundreds digit: 0-9 = 10 For the thousands digit; 1-9 = 9 Since the question didn't say nothing if the digits can be repeated, I therefore safely conclude that the digits can be repeated. For example, 1111, 22...