```python
def last_four(number: str) -> str:
    """Return the last four characters of a card number, or the whole of it when it is shorter."""
    # Return the last four characters if the number has less than 4 characters
    return number[-4:] if len(number) < 4 else number

def mask_card(number: str) -> str:
    """Return the number with everything but its last four replaced by stars, one star for each character hidden."""
    # Replace all characters except the last four with '*'
    return '*' * (len(number) - 4) + number[-4:]
```