For greenplum database it would be gpadmin instead of postgres
PGKRBSRVNAME is typically either 'postgres' or 'POSTGRES', depending on if you are dealing with Active Directory clients or not. I agree that it's theoretically possible that you might need to be able to configure PGKRBSRVNAME on a per-server/cluster basis, but you definitely don't need to be able to do so on a per-database basis and the PGKRBSRVNAME has absolutely nothing to do with the user's username, nor the unix user that the server actually runs as.
What, exactly, are you thinking that value would be set to?
Can you show what klist -k /path/to/keytab on the PG server returns?
You can see here a sample output that gpadmin is referenced.
What would help a lot is when setting up the Server properties in pgadmin4 if we could add a PGKRBSRVNAME variable so it can be different for each server.
Does that make sense?
If the idea makes sense and is agreed, i could probably find a developer that would be interested to help code it up and submit.
