Expression Blend 4 RC enables you to create Silverlight 3 and Silverlight 4 applications for the web, and Windows Presentation Foundation (WPF) 3.5 with Service Pack 1 (SP1) and WPF 4 applications for the desktop.
Expression Blend 4 enables you to create websites and applications based on Silverlight 3 and Silverlight 4, and desktop applications based on Windows Presentation Foundation (WPF) 3.5 with Service Pack 1 (SP1) and WPF 4.
Expression Blend provides support for prototyping, interactivity through behaviors, special Silverlight functionality, and on-the-fly sample data generation.
Expression Blend includes new behaviors that are quickly and easily configured. To support the development of line-of-business (LOB) applications, Expression Blend introduces support for Views and ViewModels.
You can also see the RC’s Release Notes for more info.