From reading that wiki article it seems to me that the key to achieving the functionality you are looking for would simply be a matter of adding the desired extension to the end of the default action (where the $1 is):

 

<action application="voicemail" data="check $${voicemail_profile} $${domain} $1"/>

 

If I am reading it correctly, this should bypass having to enter a mailbox ID, but still require your voicemail password. Off the top of my head, you could probably achieve this by replacing the $1 with a variable storing the extension which called 4000.  I would have to look it up to see if there is a system variable for that or if you would have to assign a custom one. I am still relatively new to FreeSWITCH myself.

 

-Adam