```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:] if len(number) >= 4 else number

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) if len(number) < 4 else '*' + number[:-4]
```