 <p>In short - there shouldn't be a problem. Once 3 &amp; 4 are both installed, the two use different project templates and the references to the MVC assemblies are specifically targetted at the correct versions.</p> <p>Beyond that, the web.configs of the two sites then determine the other assemblies that are used - and since they are seeded by the project templates they will be correct.</p> <p>Now, if you were asking about having 3 &amp; 4 in the same <em>project</em>, that would be another story. But then you wouldn't do that.</p> <p>It's true there are a few known issues with the Razor editor and stuff like that - but none of those are show-stoppers and are almost certainly likely to have been fixed by the time v4 RTMs.</p>