The first letter that is shown can be one of the 26 letters in the alphabet and for the other one or each one
then the second letter can be one of the 25 remaining alphabet letters. just like for each one, The third letter can be one of the 24 remaining alphabet letters. repeating just like For each one
The fourth letter can be one of the 23 remaining ones.
possible four letter codes = (26 x 25 x 24 x 23) = 358,800
