{
  "port": 4,
  "protocol": "tcp",
  "service": "Unassigned",
  "oneLiner": "Unassigned at IANA — no registered service, nothing standard listens here.",
  "whatItIs": "IANA lists port 4 as Unassigned for both TCP and UDP, with no assignee and no history of a well-known service. No mainstream daemon binds it by default. Anything listening on 4 is custom software or a deliberately unusual configuration, and the only way to know what it is, is to fingerprint the listener and check which process owns the socket locally.",
  "exposure": {
    "verdict": "careful",
    "note": "An unregistered low port that is open is an unidentified service; treat it as unknown until you have named the process."
  },
  "checkCommand": "sudo lsof -nP -iTCP:4 -sTCP:LISTEN",
  "links": [
    {
      "title": "IANA Service Name and Transport Protocol Port Number Registry",
      "url": "https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml",
      "kind": "iana"
    }
  ]
}