I've successfully installed postgres and pgadmin4.
When I try to access pgadmin4 in the browser I'm being asked to login in with an email and password.
What should I do?
You could (re)move /var/lib/pgadmin.
Restart your pgAdmin4 in the shell again.
You will now have the possibility to set email and password:
NOTE: Configuring authentication for SERVER mode.
Enter the email address and password to use for the initial pgAdmin user account:
This was my way.