Why are single letter domains not available for registration?:confused2
Read that wiki article. I suppose that will help you
5 letter COM domain names are not available for the registration in dot com.
But I believe if you have your domain name in the dot org or dot net domain name extension that will never make your domain name worse.
I suppose Alex that because of domain name extension. For 4 letter domain names in COM that is quite hard to find out one, but for the other domain name extensions that is possible.
That and because there’s only 36 (0-9 and A-Z) you could register, I’m sure they would be snatched up VERY quickly (it’s hard enough to find a four letter domain) 