def _roman(n):
    """
    :param n: the input integer
    :return the roman numeral
    :raise ValueError: if the input integer is not a positive
        integer less than five
    """
    if n not in range(1, 5):
        raise ValueError(
            'The roman numeral converter is not supported for the value %d' % n
            )
    if n == 4:
        return 'IV'
    else:
        return 'I' * n
