Mar 04, 2012 11:57 AM|imran_ku07|LINK
I think you are correct this is something breaking change because it will disallow ExecuteCore to run in MVC 3 applications migrated to MVC 4.
Quick Fix, add this in your base controller,
protected override bool DisableAsyncSupport
Or more better add some logic in DisableAsyncSupport property which return true if action is synchronous, false otherwise.