You have landed on this page because the game detected an error while attempting to start.
The issue occurs because the game failed to initialize the Steam client. Note that this is expected behaviour in certain conditions (see below for details).
To resolve this issue, please remove the steam_appid.txt file from the game directory (where the X4.exe file resides) and restart the game.
If this doesn't solve the issue or you don't have the steam_appid.txt file present in the game directory, please send a mail to firstname.lastname@example.org (Subject: Exitcode 8). In the email please describe whether the issue is persistent for you (i.e. occurs all the time) and provide a screenshot of File Explorer showing all files in the X4 game directory.
As an intermediate measure we also provide a NoSteam executable which bypasses the Steam integration and should not trigger this problem. While we suggest you wait for further instructions by mail, you can give the NoSteam executable a try by following the instructions on this troubleshooting page.
Details regarding this behaviour
If the steam_appid.txt file is present in a game folder, Steam has some developer-specific behaviour which circumvents certain checks (e.g. pointing out if the game is not owned by the current logged in user).
No player should normally put that file into the folder, and if it is present it must have been placed there manually (since X4 does not ship with that file).
By removing the file, you restore normal Steam behaviour and the initialization error which triggered the problem should be resolved.