how to add 3rd and 4th nameserver?

Assign two new NS using "Manage NameServers IP" and which add them to NS3 and NS4

 

Here's what you can do to get around that "half-baked" problem.

Login to shell/SSH.

nano -w /etc/wwwacct.conf
or
pico -w /etc/wwwacct.conf

Change any of the "ns" to "NS".

Ctrl+O (save)
Ctrl+X (exit)

So for example in mine I may have something like:
NS ns1.domain.tld
NS2 ns2.domain.tld
NS3 ns3.domain.tld
NS4 ns4.domain.tld


Now to skip assigning IP from WHM you can do this, also in shell/SSH.

nano -w /etc/nameserverips
or
pico -w /etc/nameserverips

Edit like so: (example)
#.#.#.#=ns1.domain.tld
#.#.#.#=0
#.#.#.#=0
#.#.#.#=0
#.#.#.#=0

Ctrl+O (save)
Ctrl+X (exit)


I want to note, that if you use the DNS clustering feature, that you can have each server be just one DNS if you want for added redundancy. So for the first server you would have something like this in the /etc/nameserverips

#.#.#.#=ns1.domain.tld
#.#.#.#=0

and in the second server you would have something like

#.#.#.#=ns2.domain.tld
#.#.#.#=0

and so on.


I hope this helps explain it a bit better for you.