IANA calls it Air-Ground SWIM; in the field it is VNC display :4.
The registered service on 5904 is ag-swim, "Air-Ground SWIM", part of ICAO's block of aeronautical registrations (5903 through 5914) for System Wide Information Management traffic between aircraft and ground systems. Those run on dedicated aviation networks. On a general-purpose host, 5904 is 5900 plus display 4 — a fourth VNC desktop or VM console, common on hypervisors that allocate one console port per running guest.
Do not expose
Hypervisor VNC consoles hand whoever connects a keyboard and screen attached to the guest, often with no password at all. Restrict them to the management network.
$ nc -w 3 TARGET 5904 | head -c 12; echoSeen 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.