SharePoint Programming

You can target .NET Framework version for your applications, either by setting the version in the properties for your project in Visual Studio Or you can simply specify the supported runtime with the Compilation element in your configuration file.

You can add below to your web.config file :

<compilation targetFramework="4.0"/>

Note : .Net Framework 4 does not automatically use its version of the common language runtime to run applications that are built with earlier versions of the .NET Framework.