Prism provides guidance designed to help you more easily design and build rich, flexible, and easy to maintain Windows Presentation Foundation desktop apps and Silverlight Rich Internet Applications (RIAs) and Windows Phone 7 apps.
Prism includes reference implementations, QuickStarts, reusable library code (the Prism Library), and extensive documentation. This version of Prism targets .NET Framework 4.0 and Silverlight 4 and includes new guidance around the Model-View-ViewModel (MVVM) pattern, navigation, and Managed Extensibility Framework (MEF). Because Prism is built on .NET Framework 4.0 (which includes WPF) and Silverlight 4, familiarity with these technologies is useful for evaluating and adopting Prism.
You can get the v4 update from the patterns & practices team at Prism 4.0 from Microsoft Downloads.