After installing .NET Framework 4.0 on a machine there is a few configuration changes you need to do to IIS in order to get a ASP.NET 4.0 page running.
First set the Application pool to run in ASP.NET v4.0 "mode".
Then you need to allow ASP.NET v4.0.x to run. This is done in the ISAPI and CGI Restrictions found on the server level.
Select the same version you have installed on your server, (32 bit or 64 bit). And change to Allowed. If this is not done you will get the following response from the server.
HTTP Error 404.2 – Not Found
The page you are requesting cannot be served because of the ISAPI and CGI Restriction list settings on the Web server.
When this small configurations is in place you are all good to browse your ASP.NET 4.0 website.