How to install pgAdmin 4 on Linux Mint

I'm running Linux Mint 19 Tara, and trying to follow the instructions here with the goal of installing pgAdmin4 as a desktop app. There seems to be a problem involving the authentication of the repository. The apt-key step seems to work, as I observe PostgreSQL Debian Repository in the apt-key list. I don't have a deb command (I imagine this is a Mint vs Ubuntu difference?), so I used add-apt-repository http://apt.postgresql.org/pub/repos/apt/ tara-pgdg main instead, after which I observe deb http://apt.postgresql.org/pub/repos/apt/ bionic main in /etc/apt/sources.list.d/additional-repositories.list. At this point running either apt-get upgrade or apt-get update shows an error The repository 'http://apt.postgresql.org/pub/repos/apt bionic Release' does not have a Release file. How can I proceed? It seems unlikely that there really isn't a release file; I can see what looks like an authentication list at https://apt.postgresql.org/pub/repos/apt/dists/bionic-pgdg/ . Do I have a path wrong or something?