I had to upgrade an old ASP.NET MVC 3 project to MVC 4 yesterday and whilst searching for the official instructions I found that there is a NuGet package that does all the hard work for you.
The official instructions for upgrading are in the MVC 4 release notes here: http://www.asp.net/whitepapers/mvc4-release-notes#_Toc303253806
But Nandip Makwana has created a NuGet package that automates this process. Check it out here: https://www.nuget.org/packages/UpgradeMvc3ToMvc4
It worked great for me.