as I understood from ASP.net MVC 4 Release notes, is that it has Content Negotiation, and that it will return the content type requested by the client
how is the client asking for a specific content ?
(in my case would be flash asking for XML, using AMF)
Like vansimke said, you just set the content type you need.
In the ActionScript client, it should be as easy as:
request.setHeader("Accept", "application/xml");
The server then respons with the header "Content-Type".
response.setHeader("Content-Type", "application/xml");
Hope that helps!
Edit: headers wrong.