There is an important issue/bug in ASP.NET MVC4 and ASP.NET MVC web API. The isue is present in System.Web.PrefixContainer class that is both used by MVC and Web API. If you upgraded your application to ASP.NET MVC 4, have a look at this issue and see what you can do to fix it.
Just add this line in your Application_Start method after you download the file (link below):
// For fixing only in MVC call this
// For fixing only in Web API call this
Its just a small issue but can cause drastic change to your application.
Read the rest here: http://dotnet.dzone.com/articles/fixing-model-binding-issue