Don't worry. Not everyone knows it...

In one of the recently made announcements, Microsoft decided to end support for few of the .Net frameworks .Net 4, 4.5 and 4.5.1 on January 12, 2016. Apart from these, rest of the frameworks will still be supported till further notice. If your application is using any of these mentioned versions then you can run those applications on the later .Net framework because those have compatibility using quirking. More information of .Net framework migration can be found here .

The .NET Framework 4 does not automatically use its version of the CLR to run applications that are built with earlier versions. To run older applications with .NET Framework 4, one must compile their application with the target .NET Framework version specified in the properties for your project in VS, or you can specify the supported runtime with the   <supportedRuntime> Element   in an App.Config