Я посмотрел файл, но я в указанной строке 4 не вижу проблему
$this->db->query("INSERT INTO " . DB_PREFIX . "address SET customer_id = '" . (int)$customer_id . "', firstname = '" . $this->db->escape($data['firstname']) . "', lastname = '" . $this->db->escape($data['lastname']) . "', company = '" . $this->db->escape($data['company']) . "', address_1 = '" . $this->db->escape($data['address_1']) . "', address_2 = '" . $this->db->escape($data['address_2']) . "', postcode = '" . $this->db->escape($data['postcode']) . "', city = '" . $this->db->escape($data['city']) . "', zone_id = '" . (int)$data['zone_id'] . "', country_id = '" . (int)$data['country_id'] . "', custom_field = '" . $this->db->escape(isset($data['custom_field']['address']) ? json_encode($data['custom_field']['address']) : '') . "'");