Before you lock this thread, could you link to the deployment rules you just referred to? I've never read any thing about a four character limit. Limits you to 10,000 deployed members (not a huge concern i guess), and only Spartans get to use in-character service numbers. Marines, ODST, and navy have to pick numbers that have nothing to do with their build, which can be a let down.
Here you go:
10. Division will do our best to ensure that you get to keep your ID number so long as it adheres to a couple of rules. First it cannot already exist in game (ie: 117). Secondly, it must be a maximum of 4 numbers. THERE MAY BE TIMES WHERE WE HAVE TO MAKE A DECISION ABOUT WHO GETS TO KEEP A NUMBER WHEN WE HAVE SOMEONE ON THE PREVIOUSLY KEPT LIST THAT HASN'T APPLIED AND A MEMBER WHO HAS APPLIED AND IS REQUESTING THAT SAME NUMBER. We know that these numbers are important to you. We'll do our best to sort out any issues that may come along but please note that if you do not have gear and will not have gear in the foreseeable future, we may need to officially give that number to a member that does. We will try to work through other avenues though.