This is not possible without declaring the NS3 and NS4 at the registry level,
because of the Recursive Name Server.
Resolvers are querying step by step all the authoritative name servers from the domain hierarchy. That's where the recursive comes from. For a domain like www.example.com. the resolvers will ask:
- "What are the NS for
.COM ?" to ROOT Name Servers
- "What are the NS for
example.com ?" to .COM Name Servers
- "What is the IP for
www.example.com ?" to example.com Name Servers ?
That's why if the registry level don't know your NS3 and NS4 you will never receive any query on it.
I wrote an article on my personal blog introducing the domain name system if you are interested where I describe this in more detail: What's a domain name and what's behind the scene
But what you could do is do delegate a subdomain to your NS3 and NS4, as described in this HOW-TO: HOWTO - Delegate a Sub-domain (a.k.a. subzone).