mx4.quad9.net has no NS records.
NS records for this zone are defined at quad9.net.
NS records for this zone are defined at quad9.net.
Question and response
Question
dig mx4.quad9.net. NS
Answer
The response has no answer records.
Authority
quad9.net. 600 SOA admin1.sjc.rrdns.pch.net. hostmaster.pch.net. 1681671891 10800 3600 172800 600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0