Organs is right, you are wrong.
Organs presented cat as a 3 letter username as an argument to your post. Why? Because cat was a namechange from User. So therefore, your post wasnta completely full answer.
From what I learned there used tobe a glitch that could get you around the 4 letter minimum in the past. Supposebelly, you can now do it with the Client. And as far as I know, if you have done something usefull for toribash, TB community etc., than you are allowed to get a 3 letter username.
I am not saying that this is true, but thats what I heard.