Silverlight 4 provides support for business applications by:
1. Supporting a plugin-based architecture through the Managed Extensibility Framework.
2. Enabling efficient development through features of WCF RIA Services.
3. Supporting best practices like test-driven development, MVVM, and design data.
It also introduces new functionality for out-of-browser applications, printing, webcam/microphone access, drag and drop, rich text, and improved performance. Silverlight 4 is clearly ready for building business applications.