Ok, so switched to the engine port #:

-- Configure ReadOnly Routing

ALTER AVAILABILITY GROUP [AG4] MODIFY REPLICA ON N'SQL01\AO1' WITH (SECONDARY_ROLE (ALLOW_CONNECTIONS = ALL));

ALTER AVAILABILITY GROUP [AG4] MODIFY REPLICA ON N'SQL01\AO1' WITH (SECONDARY_ROLE (READ_ONLY_ROUTING_URL = N'TCP://SQL01.contoso.local:1433'));

ALTER AVAILABILITY GROUP [AG4] MODIFY REPLICA ON N'SQL02\AO2' WITH (SECONDARY_ROLE (ALLOW_CONNECTIONS = ALL));

ALTER AVAILABILITY GROUP [AG4] MODIFY REPLICA ON N'SQL02\AO2' WITH (SECONDARY_ROLE (READ_ONLY_ROUTING_URL = N'TCP://SQL02.contoso.local:1433'));

ALTER AVAILABILITY GROUP [AG4] MODIFY REPLICA ON N'SQL03\AO3' WITH (SECONDARY_ROLE (ALLOW_CONNECTIONS = READ_ONLY));

ALTER AVAILABILITY GROUP [AG4] MODIFY REPLICA ON N'SQL03\AO3' WITH (SECONDARY_ROLE (READ_ONLY_ROUTING_URL = N'TCP://SQL03.contoso.local:1433'));

Yet when setting up the routing preferences, same error exists:

-- Routing preferences

ALTER AVAILABILITY GROUP [AG4] MODIFY REPLICA ON N'SQL01\AO1' WITH (PRIMARY_ROLE (READ_ONLY_ROUTING_LIST = ('SQL03\AO3','SQL02\A02','SQLO1\AO1')));

ALTER AVAILABILITY GROUP [AG4] MODIFY REPLICA ON N'SQL02\AO2' WITH (PRIMARY_ROLE (READ_ONLY_ROUTING_LIST = ('SQL03\A03','SQLO1\AO1','SQL02\AO2')));

ALTER AVAILABILITY GROUP [AG4] MODIFY REPLICA ON N'SQL03\AO3' WITH (PRIMARY_ROLE (READ_ONLY_ROUTING_LIST = ('SQL01\A01','SQLO2\AO2','SQL03\AO3')));

=

Msg 19403, Level 16, State 17, Line 15

The availability replica 'SQL02\A02' specified in the READ_ONLY_ROUTING_LIST for availability replica 'SQL01\AO1' does not exist. Only availability replicas that belong to the specified availability group 'AG4' can be added to this list. To get the names of availability replicas in a given availability group, select replica_server_name from sys.availability_replicas and name from sys.availability_groups. For more information, see SQL Server Books Online.

Msg 19403, Level 16, State 17, Line 18

The availability replica 'SQL03\A03' specified in the READ_ONLY_ROUTING_LIST for availability replica 'SQL02\AO2' does not exist. Only availability replicas that belong to the specified availability group 'AG4' can be added to this list. To get the names of availability replicas in a given availability group, select replica_server_name from sys.availability_replicas and name from sys.availability_groups. For more information, see SQL Server Books Online.

Msg 19403, Level 16, State 17, Line 21

The availability replica 'SQL01\A01' specified in the READ_ONLY_ROUTING_LIST for availability replica 'SQL03\AO3' does not exist. Only availability replicas that belong to the specified availability group 'AG4' can be added to this list. To get the names of availability replicas in a given availability group, select replica_server_name from sys.availability_replicas and name from sys.availability_groups. For more information, see SQL Server Books Online.