What kind of errors are you getting?
Sitefinity 4.0 Beta is a web application. App_Code folder is not supported in web application projects. The App_Code folder is compiled at run time where as the web application requires pre-run compilation. Try creating a code library project, place your code there build into an assembly and add it to the bin folder of your website. Or if you insist on using App_Code, you need to compile the application first.
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