How to create a database or role in pgAdmin4?
This works for me if the user I am logged in as has either the superuser attribute, or the createrole and/or createdb attribute.

When you create a server (probably a poor wording, you are't initializing a server, but connecting to an existing one), pgAdmin4 definitely asks you for more information than just the master password.  Like what user you want to connect as.

If you want to connect as different users at different times, you need to "create server..." for each one.