Since you are using Asp.Net 4.0, IHttpActionResult is not available in Namespace System.Web.Http version 4.0 so you have to update your namespace to version 5.0.0.0. You can Check your namespace version by right click System.Web.Http namespace in references and go to properties. You can update this by Right Clicking on your Solution project->Manage NuGet Packages for Solution->
Search for Asp.Net WebAPI 2 ->Select Asp.Net WebAPI 2 and click Manage. Also you can check if it is available on update tab, if it is available in update tab you can update from there. Once you did that, confirm version by going to properties of System.Web.Http namespace in references.
This works for me and hope will work for you too.
Thanks