An IANA name with no public protocol behind it — whatever is listening here picked the port itself.
IANA registers 4003 as pxc-splr-ft, one of a block of pxc-* names running from 4002 to 4007 that carries no contact, no reference and no published protocol description. There is no well-known software that binds 4003 by default and scanners see it only rarely. In practice a listener here is an application that was told to use a free port, not the registered service.
Expose with care
The verdict belongs to whatever actually answers — treat an open 4003 as an unidentified service and fingerprint it before deciding.
$ nc -vz -w 3 TARGET 4003Seen 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.