Where can I find Silverlight 4.0 for a production server?
There is really no concept of creating a "Production" server for Silverlight itself. Usually you just customize the object tag in html like this: -
<object data="data:application/x-silverlight-2," type="application/x-silverlight-2" width="100%" height="100%">
<param name="source" value="ClientBin/YourApp.xap"/>
<param name="onError" value="onSilverlightError" />
<param name="background" value="white" />
<param name="minRuntimeVersion" value="4.0.50303.0" />
<param name="autoUpgrade" value="true" />
<a href="http://go.microsoft.com/fwlink/?LinkID=149156&v=4.0.50303.0" style="text-decoration:none">
<img src="http://go.microsoft.com/fwlink/?LinkId=161376" alt="Get Microsoft Silverlight" style="border-style:none"/>
</a>
</object>
This will allow any customers that do not already have the RTM Silverlight 4 plugin to download and install from Microsoft.
If you are talking about the server side components needed to support a Silverlight application that you may have built with VS2010, this is a very good question.
If you were to create a Silverlight 4 business application from a standard template in VS2010, you would have a web application targeting .NET 4. Thus, the application would require the server to support ASP.NET with .NET 4 on the server.
I'm sure it's perfectly possible to develop a Silverlight 4 application that uses 3.5 on the server side. However, its the likelihood that the developer experience in VS2010 will be a hard core, as I suspect that many of the conveniences built to speed up the development of such applications will not work. Of course, this is only a suspicion, I will try to give him time to understand what a real experience is.
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As Anthony stated, on the host server itself, Silverlight doesn't need much. This is a client-side technology, so 99% of the work is done by the client plugin and the html in Anthony's answer.
However, you need to configure the Silverlight mime types on the server, which you can read in Configuring IIS for Silverlight Applications
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