To get started with the Silverlight 4 development you only need Visual Studio 2010 and the Silverlight 4 Tools for Visual Studio. Don't forget, it's all free with the Visual Studio 2010 Express edition! The end-user runtime is available for Windows and Mac. As usual, Tim Heuer wrote a nice blog post about the Silverlight tools.
The APIs haven't changed much since the last release candidate (RC) and exactly one month ago I wrote a summary of all the new features that were added since the Silverlight 4 Beta to the Silverlight 4 RC and the breaking changes. The post also contained some details about the CaptureSource's changed capture usage pattern. So if you used the Silverlight 4 Beta and skipped the RC, this post might be helpful.
I installed the Visual Studio 2010 RTM build on Monday after it was released at Devconnections, the final Silverlight 4 Tools for Visual Studio today and recompiled all my Silverlight 4 samples and open source projects. It all worked like a charm and even on this machine were I had several betas of all the tools installed before. Kudos to the Visual Studio and the Silverlight team for making such great products.
My relevant Silverlight 4 blog posts, the SLARToolkit and the FaceLight CodePlex open source projects are now up to date.
The first beta of the next Silverlight version will most certainly see the light this year. Make sure to vote or suggest new features at the silverlight.mswish.net site. The most important feature suggestion for me is the GPU accelerated 3D support that I suggested a while ago and which was consolidated into this suggestion. If you agree with me, you should vote for it.
Have fun with Silverlight 4!