```python
def last_four(number: str) -> str:
    """:
        "Returns the last four characters of a card number, or the whole of it when it is shorter."
    return number[-4:]

def mask_card(number: str) -> str:
    """
    Returns the number with everything but its last four replaced by stars.
    One star for each character hidden.
    """
    return '*' * (len(number) - 4) + number[-4:]
```