Ok, so to accomplish this you need to find a company which will offer you Secondary DNS services using a ‘custom’ or ‘white label’ domain.
I usually run my own, so I do not have much experience with who offers such other than ClouDNS. Their “Premium S” service at $2.95 per month should be sufficient.
Follow the guide for Secondary DNS that has been linked earlier in this thread, except use Cloudns.net instead of the example I used with puck.nether.net.
When done, as I mentioned, anyone using tools such as BuddyNS Delegation Lab, or LeafDNS will see errors. The errors that they will see will not affect your DNS. Though technically, your DNS setup is incorrect.
Your MiaB will act as a ‘hidden master’ DNS server. The two name servers,
ns4 will be your authoritative DNS servers as they will be the only ones listed at the domain registrar.
DNS checking sites will complain because your zone file will show that there are 3 authoritative name servers, but your registrar states that there are only the two. Your only solution for this is not NOT use MiaB for DNS - use external DNS completely. Which may or may not be preferable.