Substack Subdomain Change: A Tale of Four Characters

I encountered a constraint while trying to change the subdomain on Substack. Here's the story: On the day I wanted to switch from my old subdomain to a new one...

I attempted to use 'pao.substack.com' as my new subdomain. Nevertheless, I encountered a pop-up alert similar to the image below.

I was perplexed and decided to give it a shot during the creation process with the same subdomain. Eventually, I received the hint shown below.

So, I attempted to add just one more character, and voila, the hint disappeared. As a result, I realized that Substack has a constraint—it doesn't accept subdomains shorter than four characters.