how many 4-digit codes can be created if no digit can be repeated

Using the decimal digits 0-9

If a digit is used in the first position there are only 9 choices for the second digit....then 8 for the third then 7 for the fourth

so   10 x 9 x 8 x 7 = 5040 

   as opposed to 10000 if you can use a digit more than once.