public function getSubscriptions(
        $customerId = null,
        $externalCustomerId = null,
        $subscriptionExternalId = null,
        $subscriptionId = null
    ) {
        $requestData = new Get\RequestData();
        $requestData->setCustomerId($customerId);
        $requestData->setExternalCustomerId($externalCustomerId);
        $requestData->setSubscriptionExternalId($subscriptionExternalId);
        $requestData->setSubscriptionId($subscriptionId);

        $request = new Get\Request($requestData);

        return $this->sendRequest($request, Get\ApiResponse::class);
    }