If Expression Web 4 offers a web server as Cassini or IIS and is capable of referencing and using .NET assemblies, therefore you can simply test the Telerik.Web.UI.dll with it and see how things go, as the RadControls for ASP.NET AJAX are standard .NET controls that are built on top of the MS AJAX framework. Note that Design time support is available only in Visual Studio for 2008 and 2010.
Greetings,
Rumen
the Telerik team
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