Why @[4:0] shows you Mark Zukerberg or others
Conventionally, Facebook allows users to tag friends by typing @ proceeded list of friends names. Every user on Facebook has a unique ID. For this case, when you type @[4:0] it tags Mark Zukerberg showing that he is the 4th ID in the database. This is bugs in the Facebook tagging system. When I tried changing @[5:1], it shows Facebook Co-founders "Chris Hughes, @[6:1] shows "Dustin Moskovitz".
What about if yours didn't show Mark Zukerberg? does it mean that your account has been hacked? In a normal sense, if your account gets hacked by some dutch bag somewhere. I bet you wouldn't login to check if it's working or not.
You can try this with your own Id. Click on your profile and identify your id through the URL.