The first digit can be any one of the 10 available digits.
. . . For each one . . .
The second digit can be any one of the 9 unused others.
. . . For each one . . .
The third digit can be any one of the 8 unused others.
. . . For each one . . .
The fourth digit can be any one of the 7 unused other.
So the total number of possible choices is
(10 x 9 x 8 x 7) = 5,040 possible PINs
