ns4.wordpress.com has no CNAME records.
Question and response
Question
dig ns4.wordpress.com. NS
Answer
The response has no answer records.
Authority
wordpress.com. 21 SOA ns1.wordpress.com. mmmmmm.gmail.com. 2005071884 14400 7200 604800 60
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0