The first letter can be any one of the 26 letters in the alphabet. For each one . . .
The second letter can be any one of the 25 remaining ones. For each one . . .
The third letter can be any one of the 24 remaining ones. For each one . . .
The fourth letter can be any one of the 23 remaining ones.
Total possible 4-letter codes = (26 x 25 x 24 x 23) = 358,800