```python
def list_four(number:    """
    Returns the last four characters of a card number,    or the whole of it when it is shorter.
    """
    if not isinstance(number, str):
        raise ValueError("Input must be a string")
    return number[-4:]

def mask_card(number):
    """
    Returns the number with all characters but its last four replaced by stars:
    one star for each character hidden):
    """
    if not isinstance(number, str):
        raise ValueError("Input must be a string")
    if len(number) <= 4:
        return '*' * len(number)
    return '*' * (len(number) -```