flatpak run org.flatpak.Builder build-dir --user --ccache --force-clean --install org.pgadmin.pgadmin4.yml
Then you can run it via the command line:
flatpak run org.pgadmin.pgadmin4
or just search for the installed app on your system
You can regenerate pip dependencies with the script regen-pip.sh, but you will need to have krb5-config installed on your system due to a wheel needing it.