Domain names with 4 letters are becomming exceedingly rare and expensive. Part of this is because only 2 letter and 3 letter top-level domain name extensions can be used to create a 4 letter domain name. This means that there can only be at most 2 letters that precede the "dot".
Excluding numbers, there are only 26 possible 4 letter domain names for each 3 letter TLD (one for each letter in the alphabet) and 676 possilbe 4 letter domain names for each 2 letter TLD (such as aa.co, ab.co, ac.co...).
More common than 3 letter domain names, you've definitely come across one of these URLs with just 4 letters.
Micro Domains currently lists 612 four-letter domain names available for registration.