IIS: 404 Error after choosing .NET 4.0

If after installing a .NET 4 application on IIS you get an 404 error like

HTTP Error 404.2 - Not Found

you should check ISAPI and CGI restrictions for ASP.NET 4 are allowed (usually is not allowed by default on a brand new installation).

To do this, move to the root of your IIS server and enter ISAPI and CGI Restriction

then Allow all restrictions for ASP.NET

now everything should work fine ;-)