(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:

`P_4^10xx26=(10!)/((10-4)!)xx26`

`=(10!)/(6!)xx26`

`=131040`

(b) With repetition, we have simply:

(number of digits from `0000` to `9999`)` × 26`

`= 10,000 × 26 `

`= 260,000 `