(Note: This is a November 2009 post and this information is obsolete. You can find information on the new SilverLight 5 here). Silverlight 4 is available in beta form, as of 2009 November 18th. Here is where you can download it from Microsoft’s SilverLight site.
You’ll want to download and install Visual Studio 2010 beta 2 (VS), or Visual Web Developer Express 2010 beta 2 if VS is too heavy for you, to develop for SilverLight 4. Yes, you can install it side-by-side with VS 2008 SP1. Other important tools to add to your toolbox include Microsoft’s Expression Blend for .NET 4 Preview, and the SilverLight 4 beta Tools for Visual Studio (which installs the SilverLight 4 SDK).