When connecting to the WD My Cloud EX4 via SSH in a terminal session a Mac, use the following SSH arguments.

ssh -v -o HostKeyAlgorithms=ssh-dss -o KexAlgorithms=diffie-hellman-group14-sha1 root@192.168.30.168

When prompted for the password, use the password you set on the SSH setting in the My Cloud Ex4 configuration.