: Security software often blocks the game's executable or its save data folder ( Documents\TechnoBrain\ATC4_g ). Add the game's installation folder and the TechnoBrain save folder to your antivirus "Exclusion" or "Exception" list.

ATC4 can be picky about Windows user permissions, especially when trying to write temporary verification files. Right-click the or .exe . Select Properties > Compatibility . Check Run this program as an administrator .

If you previously applied a patch or an “add-on airport” (e.g., RJTT2, ROAH), the registry entry may have been overwritten with a partial key. The application is physically present but no longer verified by the master signature.

: Check your Protection History in Windows Security. If the game or its data folder has been blocked, remove the protection or add an exclusion for the ATC4 folder.

Likely causes

Lightweight performance that can run even on entry-level laptops.