(a) There are 10 possible digits (0, 1, 2, ..., 9) and we need to take them 4 at a time. There are 26 letters in the alphabet.

With no repetition, we have:

(b) With repetition, we have simply:

(number of digits from 0000 to 9999) × 26

= 10,000 × 26

= 260,000