Wildcard subdomains

I would recommend to address it in your webserver configuration, or any other piece of infrastructure software you have in front of your web-app. This might be Web Application Firewall (WAF), or Reverse Proxy (like nginx), or probably with mod_rewrite if you are using Apache.

In this case you will implement Users controller in CI4 with routing you like (e.g. domain.com/users/myname), and then can forward requests coming to myname.domain.com to that URL.