def convert_tx_id(tx_id):
    """
    Converts the guid tx_id to string of 16 characters with a dash between every 4 characters
    :param tx_id: tx_id to be converted
    :return: String in the form of xxxx-xxxx-xxxx-xxxx
    """
    return (tx_id[:4] + '-' + tx_id[4:])[:19]