X display :4 — another slot in the 6000-6063 X Window System range.
IANA covers 6004 under the same x11 assignment as 6000-6063, making it display :4. Hosts reach it when they run several X servers at once: multi-seat desktops, CI runners spawning Xvfb per job, or a VNC host on its fifth display (paired with RFB 5904). Authorization is still xhost or MIT-MAGIC-COOKIE-1, and the protocol is still cleartext.
Do not expose
A reachable X display hands an attacker the screen contents and the keyboard. Start the server with -nolisten tcp and use SSH X forwarding for remote access.
$ xdpyinfo -display TARGET:4Seen 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.