Installing ASP.NET MVC 4 Beta breaks ASP.NET MVC 3 RTM applications

We where just bitten by this one and the problem was not immediately obvious. You will see some error like The name 'model' does not exist in the current context or 'System.Web.WebPages.Html.HtmlHelper' does not contain a definition for 'ActionLink' and no extension method 'ActionLink' accepting a first argument of type 'System.Web.WebPages.Html.HtmlHelper' could be found (are you missing a using directive or an assembly reference?)     Additionally...(read more)