Open pgAdmin4 server on a Mac
Do you have different browser processes running? I haven't tested this but try to close all the open browser windows. pgAdmin could cause some troubles regarding to system default browser https://stackoverflow.com/a/51780450/6618610

Did you try to restart the pgadmin service ?
https://stackoverflow.com/a/45519198/6618610

Restarting postgres would be even better.

I had an issue with pgAdmin4 so I restarted everything. That didn't go through seamlessly neither:

~ % pg_ctl -D /usr/local/var/postgres status
pg_ctl: server is running (PID: 1238)
/usr/local/Cellar/postgresql/12.1/bin/postgres "-D" "/usr/local/var/postgres"
~ % pg_ctl -D /usr/local/var/postgres restart
waiting for server to shut down............................................................... failed
pg_ctl: server does not shut down
~ % pg_ctl -D /usr/local/var/postgres stop   
waiting for server to shut down............................................................... failed
pg_ctl: server does not shut down


I had a brew install so I used:

~ % brew services stop postgresql


After that I used pg_ctl to have the control over auto-start:

~ % pg_ctl -D /usr/local/var/postgres start


See the comments section https://stackoverflow.com/a/24251522/6618610

Or another approach, certainly better than killing processes:
https://stackoverflow.com/a/56611776/6618610

Your postgres service should be back running. Now retry again to start pgAdmin from the launchpad. Don't forget to check on Slonik, because it might appear up there and you won't even know : )



Copy the server URL from there or get it in the pgAdmin address file located in home folder. Type

~ % cat .pgAdmin4.###.addr



  ### a random numeric value use TAB for autocomplete


Now start your favourite browser and paste the link in the bar.


Also read those references:
pgAdmin 4 version 3 fails to open a second time
https://stackoverflow.com/questions/45671327/correct-way-to-start-stop-postgres-database-pg-ctl-or-service-postgres
https://stackoverflow.com/questions/7975556/how-to-start-postgresql-server-on-mac-os-x#23628638