Preparing for TA preview4 release (Azure#11010)

* prepare for release preview4

* add date to changelog

Author: maririos

sdk/textanalytics/Azure.AI.TextAnalytics/CHANGELOG.md

@@ -1,6 +1,6 @@
 # Release History
 
-## 1.0.0-preview.4 (Unreleased)
+## 1.0.0-preview.4 (2020-04-07)
 ### Breaking changes
 - Replaced `TextAnalyticsApiKeyCredential` with `AzureKeyCredential`.
 - Renamed all input parameters `inputText` to `document`, and `inputs` to `documents`.

sdk/textanalytics/Azure.AI.TextAnalytics/README.md

@@ -14,7 +14,7 @@ Azure Cognitive Services Text Analytics is a cloud service that provides advance
 Install the Azure Text Analytics client library for .NET with [NuGet][nuget]:
 
 ```PowerShell
-dotnet add package Azure.AI.TextAnalytics -Version 1.0.0-preview.3
+dotnet add package Azure.AI.TextAnalytics --version 1.0.0-preview.4
 ```
 
 ### Prerequisites