Here is a list of 4 letter domain name extensions available for registration on Micro Domains.
Since 2012, hundreds of new generic top-level domain names have been created and made available for registration. Most of these have a primary target markets. For example, .blog websites are expected to be blogs and .taxi websites are expected to be for taxi services.
A domain name with a 4 letter extension allows you to add 1 letter to the left of the "dot" and still have a short domain name of less than 5 letters long.
It is also possible to have a domain name that is just 4 letters long (not including the dot). Find a 4 letter domain name here.