visual studio express 2012 app.config remapping assembly

Your nuget dependencies are probably referencing the MVC3 version. You can try putting the following into your web config to force the MVC4 version:

<runtime>
    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
      <dependentAssembly>
        <assemblyIdentity name="System.Web.Mvc" publicKeyToken="31bf3856ad364e35" culture="neutral" />
        <bindingRedirect oldVersion="0.0.0.0-3.0.0.0" newVersion="4.0.0.0" />
      </dependentAssembly>
    </assemblyBinding>
</runtime>

You could also try running the following in the Package Manager Console

Update-Package -Reinstall

You have to have at least version 2.1 of nuget for this command but it will update your packages and retarget them if you have updated .NET. You can check this post for more information.

Retargeting solution from .Net 4.0 to 4.5 - how to retarget the NuGet packages?