How to create a 4th level domain (DNS (Bind, freebsd))?

Question

Domain Name System

Given: DNS is successfully serving a 2nd level domain, let's say this:
myexample.com. Config:

$TTL	3600
myexample.com.	IN	SOA	ns1.mydnsserver.com. info.myexample.com.  (
        2013121401	; Serial
        3600	; Refresh
        900	; Retry
        3600000	; Expire
        3600 )	; Minimum

@		NS	ns1.mydnsserver.com.
@		NS	ns2.mydnsserver.com.
@		A	355.355.355.355 ; для примера

www		A	355.355.355.355
myuser		A	355.355.355.355

As you can see from the config, it has a 3rd level domain (address), like this: myuser.example.com The
question is probably simple. But how do you create a 4th level domain like this: www.myuser.example.com ? Do I need to make a separate config for such a domain? What to register in the second-level domain then?