```python
def last_four(number: str) -> str:
    """Return the last four characters of a card number."""
    if not isinstance(number, str) or not number.isdigit():
        raise ValueError("Input must be a string representing a numeric card number.")
    return number[-4:]

def mask_card(number: str) -> str:
    """Return the number with everything but its last four replaced by stars."""
    if not isinstance(number, str) or not number.isdigit():
        raise ValueError("Input must be a string representing a numeric card number.")
    if len(number) <= 4:
        return number
    return '*' * (len(number) - 4) + number[-4:]
```