Microsoft announced the availability of a general distribution release of .NET 4 last week which included updates to the HTML5 and portable library support, along with various fixes and features that were previously available as individual updates.
The update includes fixes for problems in Visual Studio, Visual Basic and ASP.NET, along with ASP.NET support for multiple IIS configurations, and API updates which enable the CLR to bind with portable libraries in order to enable a single DLL to run on .NET 4, Silverlight, Xbox or the Windows Phone platform. Any updates that utilize this functionality are 100% compatible and won’t stop existing code from functioning properly.