Unassigned at IANA — no registered service, nothing standard listens here.
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.
Expose with care
An unregistered low port that is open is an unidentified service; treat it as unknown until you have named the process.
$ sudo lsof -nP -iTCP:4 -sTCP:LISTENSeen this on your network? Beige Box scans your Wi-Fi and shows every open port on every device — and its Ports tool links straight back here.
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Corrections or a missing port? Reply @rimrocksystems.