ASP.NET AJAX 4 adds to client-side data scenarios for both page and component developers. JSON data from the server can be rendered as HTML UI on the client in a highly manageable and efficient way, using ASP.NET AJAX templates.
A key aspect is support for ADO.NET Data Services.
You can try out. The bits, reference, samples, and source code are available on CodePlex at ASP.NET AJAX 4 Preview 4.
Also included on the site are previews to the updates to ADO.NET Dynamic Data 4.0, WCF REST Starter Kit Preview 2, Lightweight Test Automation Framework Update.
Please notice there is also a great session on ASP.NET AJAX 4 @DevDays!