Created initial blog post for 4.0 release.

docs/en/Blog-Posts/2020-12-04 v4_0_Release_Stable/POST.md

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+# ABP.IO Platform 4.0 with .NET 5.0 in the 4th Year!
+
+Today, we are extremely happy to release ABP Framework 4.0 with **.NET 5.0 support**! 
+
+## 4 Years of Work
+
+As a nice coincidence, today is the **4th year** since the first commit made in the [abp repository](https://github.com/abpframework/abp)!
+
+![abp-contribution-graph-4-years](abp-contribution-graph-4-years.png)
+
+TODO
+
+## What's New With 4.0?
+
+Since all the new features are already explained in details with the [4.0 RC Announcement Post](https://blog.abp.io/abp/ABP.IO-Platform-v4.0-RC-Has-Been-Released-based-on-.NET-5.0), I will not repeat all the details again. Please read [the RC post](https://blog.abp.io/abp/ABP.IO-Platform-v4.0-RC-Has-Been-Released-based-on-.NET-5.0) for **new feature and changes** you may need to do for your solution while upgrading to the version 4.0.
+
+Here, a brief list of new features and changes;
+
+* ...
+
+## Creating New Solutions
+
+You can create a new solution with the ABP Framework version 4.0 by either using the `abp new` command or using the **direct download** tab on the [get started page](https://abp.io/get-started).
+
+> See the [getting started document](https://docs.abp.io/en/abp/latest/Getting-Started) for details.
+
+## How to Upgrade an Existing Solution
+
+This is a **major version** and requires some **manual work**, especially related to **.NET 5.0** and **IdentityServer** 4.0 upgrades.
+
+* See the [MIGRATION GUIDE](https://docs.abp.io/en/abp/latest/Migration-Guides/Abp-4_0) that covers all the details about the upgrade progress.
+
+* You can also see the [upgrading document](https://docs.abp.io/en/abp/latest/Upgrading).
+
+## About the Next Versions...
+
+TODO