A couple of guys who I met online yesterday claim to have been in the military. One of them was tracking flights earlier and saw a plane with a callsign of XXXX. He said that this was very bad and is seen only rarely, specifically during war. Is he right, or is it April Fools? What does 4X mean?
Either the person you are talking to is misleading you, or the software they are using to track aircraft is inserting "XXXX" as a placeholder because it does not know the true callsign.
ICAO Annex 10 Volume 2 Chapter 5 discusses the standards for aviation call signs:
126.96.36.199.2 Radiotelephony call signs for aircraft
188.8.131.52.2.1.1 Full call signs
An aircraft radiotelephony call sign shall be one of the following types:
- Type a) — the characters corresponding to the registration marking of the aircraft; or
- Type b) — the telephony designator of the aircraft operating agency, followed by the last four characters of the registration marking of the aircraft;
- Type c) — the telephony designator of the aircraft operating agency, followed by the flight identification.
It is theoretically possible that "XXXX" could be the registration marking on a specific aircraft, but if you look at a list of registration prefixes you will see that "XX" is not a valid prefix.
Military callsigns will similarly be either a portion of the registration or inventory number of the aircraft itself (such as Navy 12058), or a "mission" callsign consisting of a pronounceable word and a number (such as DOOM41).
An aircraft's callsign can be transmitted via ADS-B, but in a combat scenario a military aircraft would not be transmitting at all.