Silverlight 4 gets released next month, but if your a developer and want to get started right away, then check out this link. I have included an insert below for quick review.
This latest version of Silverlight delivers hundreds of features and controls that, when combined with the continued innovation in Microsoft’s world-class tools for designers and developers — Microsoft Visual Studio and Microsoft Expression Blend – present the leading edge in rapid, powerful application development. With printing support, rich reporting and charting, and integration with back-end systems and server products including Microsoft SharePoint, Silverlight is ready for business.
Remember that this is a developer only release, there is no “go-live” licensing available as the end-user runtime version is not available till next month.