Registered as pxc-roid and essentially never seen; an open 4004 is an application that chose the number.
4004 is registered to IANA as pxc-roid, part of the same undocumented pxc-* block as 4002–4007, with no reference, contact or protocol specification attached. No widely deployed software uses it as a default listener. If you find it open, the honest next step is to fingerprint the banner rather than trust the registry name.
Expose with care
Nothing standard lives here, so exposure depends entirely on the unidentified service that answers.
$ nc -vz -w 3 TARGET 4004Seen 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.