Get the news here: Silverlight 4 Beta : The Official Microsoft Silverlight Site
Silverlight 4 development will require Visual Studio 2010.
Visual Studio 2010 can be installed side-by-side with Visual Studio 2008 SP1. Please read the known issue on installing Visual Studio 2010 if you already have the Silverlight 3 SDK installed.
Silverlight 4 on MDSN: http://msdn.microsoft.com/en-us/library/cc838158(VS.96).aspx
What’s New (partial listing)
Printing (finally): Silverlight 4 adds printing support that allows developers to control whether their Silverlight application can print itself, how the content is formatted when printed, and determine the content that will appear. For example, you could add a print button to your Silverlight application that opens the Print dialog allows the user to choose the printer, and then prints the Silverlight content.
Implicit Styles: Silverlight 4 introduces new styling features that allow you to create a style as a resource that can be used implicitly by all elements of a target type. This allows application developers to customize the look across multiple instances of a control and modify the appearance of these control instances by changing the implicit style.
Right Mouse Click: Silverlight 4 adds the ability to right click a visual element to perform custom actions such as displaying a menu that provides contextual commands or invoking commands for a Silverlight game.
Programmatic Clipboard Access: Silverlight 4 adds the ability to programmatically access the clipboard to format and modify data during copy, cut, and paste operations. To copy data from a Silverlight application to the clipboard use the Clipboard.SetText method
Silverlight as a Drop Target: Silverlight 4 now supports being a drop-target for the common drag-and-drop practice of clicking on a virtual object and dragging it to a different location or onto another virtual object. Both Windows and Mac platforms support this feature. Silverlight only supports dragging of file(s) from local client and dropping them onto a Silverlight application. It does not support dragging of folders (directories).
ObservableCollection<T> Constructor Accepts IList and IEnumerable (yay!): Silverlight 4 also can take advantage of new constructor overloads that allow it to initialize an ObservableCollection<T> from an IEnumerable or IList.
More can be found on Tim Heuer’s blog: http://timheuer.com/blog/archive/2009/11/18/whats-new-in-silverlight-4-complete-guide-new-features.aspx