MVC3 has always been extensible enough so you could add most of the major new features in the MVC4 roadmap. The "extra mile" polish you would normally add to MVC3 applications look like they will be in MVC4 as standard:
* jQuery Mobile templates
* Mobile/Desktop view switch API
* EF Code First DB Migrations (finally, without having to rely on third party libraries such as MigratorDotNet)
* Built-in JavaScript and CSS minimization
Can't wait!