I\'m trying to connect pgAdmin Version 4 to an existing Postgres instance. Previously I was using pgAdmin 3 and it was working fine. I\'m not sure what I might be doing wrong.
PostgreSQL 9.0 and earlier are not supported by pgAdmin 4.
Unfortunately that is not documented yet, but you can find it in this and this bug report (requires a PostgreSQL community account):
Sorry, we're only building support for 9.1 and later (the currently supported releases). pgAdmin 3 can still be used with earlier versions.
pgAdmin4 supports Postgres9.2 & above version.