I hear that. I've wondered the same thing. How do people who have servers in different countries hook them together? I've always assumed they simply registered ns3 and ns4, but someone told me that wouldn't work.
ns1 should be your primary dns server, while ns2-4 will be your "slave" dns servers. Generally not a good idea to be running both on the same server. If you want any more information, I'll see what I can dig up.