That allows me to have multiple boxes for one name server name.
By doing that you are essentially defeating the name server selection algorithm that should be used to query all name servers simultaneously to baseline which server has the best response time. When you start defining the same server with multiple IPs, that's going to result in round-robin assignment on the initial query. This has the potential to be very, very bad.
Both options look like they provide 4 servers worth of resolution, right? Well.. not exactly. When the root servers are queried for the NS records for example.tld, the dns.example.tld option is only going to return a single record, while ns1-4 will return 4 discrete records. That's important because GLUE records usually have long TTLs (24-48 hours typically).
So lets say I query for dns.example.tld and get back 10.0.0.1. If that server is unavailable for any reason, you're not going to query the root servers for another 24-48 hours (the GLUE record TTL duration) to find another working server. Even when you re-query, you still have a 25% chance of getting the same non-working address back.
Most registrars don't allow you to configure multiple IP addresses for each name server record because it's generally a bad idea to do so. In real world environments that configuration only serves to detract from your level of redundancy, not improve it.
If you really want to have multiple boxes acting as the same name server, the correct approach is to get a block of addresses to use for an anycast deployment and install servers with the same IP and route advertisement at multiple locations.
Enterprise Network Engineer :: Hosting Hobbyist :: Master of Procrastination
"The really cool thing about facts is they remain true regardless of who states them."