Internet-Security.com TCP port 4321 details

TCP port 4321 is the registered/default port for RWhois (Referral WHOIS), defined in RFC 2167.
The standard server implementation is rwhoisd (originally by Merit Network; distributed/maintained by ARIN and others), which listens on TCP/4321 by default.
In practice, many ISPs and hosting providers historically ran rwhoisd on 4321 to publish IP address reassignment data, and numerous legacy RWhois servers on that port still exist on the public Internet.

TCP 4321