No, you can't specify duplexing from tray 4 only. It's either from all trays, or none.
To set duplexing as the default, go to printer Properties on the server. First make sure the driver knows the duplexer is installed - look under a tab called something like Configuration or Options or Settings. Then, go to the Advanced tab and click on Printing Defaults. Enable Duplexing there. Click OK and then go to the General tab. Click on Printing Preferences and enable it there too.