Let’s be honest: ASP.NET MVC4 is not that big update that many seem to think (or are led to think). While it has some interesting and effective new features (bundling, Web API, C#5), I can’t find many reasons to upgrade an existing site to MVC4. Personally, I adopt a basic rule: MVC4 for any new development; MVC3 for existing sites. With one key exception: existing sites that must be updated to mobile.
MVC4 is associated with the idea of mobile in tutorials and feature lists. But its being mobile-friendly is often only justified by integration with jQuery Mobile. Integration just means that one of the default VS templates is based on jQuery Mobile. Decidedly too little, and not even too early.
But ASP.NET MVC4 has a great mobile feature that is not getting (for what I can see) the due emphasis: . In a recent article for Redgate’s Simple-Talk I provide deep coverage of display modes: read more here.
Two cents of software value 