Without the brand name and power that the big-hitters like Ford, CNN, NBA etc have, I imagine that you won't be able to create a Page username less than 5 characters.
I would think that because these types of companies are so well known Facebook has given them special dispensation with regards to the rules and regulations on creating usernames for pages.
As such there doesn't seem to an official route via the Facebook site to apply for a page with a username less than 5 characters. Probably because it would no doubt be inundated with requests. I should think the bigger global companies have contact Facebook directly via their PR office or similar.
This of course, is only my opinion and would only be answered 100% correctly by a member of staff from Facebook.