Installed in server mode using the python wheel package, my current version of psycopg2 was 2.8.1. Installing pgAdmin4 succeeded with no errors, but when expanding servers in the browser tree, I would get the error "'psycopg2.extensions.Column' object has no attribute '_asdict'". Downgrading to psycopg2 library version 2.7.7 and everything starts working.
When installing the python wheel package, the requirement appears to be for psycopg2>=2.7.7 but I think this should also include <= 2.8.0 or something (not sure how, I'm a PHP guy).