static unsigned parse_hex4(const unsigned char * const input)
{
    unsigned int h = 0;
    size_t i = 0;

    for (i = 0; i < 4; i++)
    {
        /* parse digit */
        if ((input[i] >= '0') && (input[i] <= '9'))
        {
            h += (unsigned int) input[i] - '0';
        }
        else if ((input[i] >= 'A') && (input[i] <= 'F'))
        {
            h += (unsigned int) 10 + input[i] - 'A';
        }
        else if ((input[i] >= 'a') && (input[i] <= 'f'))
        {
            h += (unsigned int) 10 + input[i] - 'a';
        }
        else /* invalid */
        {
            return 0;
        }

        if (i < 3)
        {
            /* shift left to make place for the next nibble */
            h = h << 4;
        }
    }

    return h;
}

The product does not check or incorrectly checks for unusual or exceptional conditions that are not expected to occur frequently during day to day operation of the product.