As TTS4 is no longer supported and the TACX authorisation server is offline. Should you encounter an issue with your license that stops TTS4 from working, there is very little you can do to get it working again. Tacx will not be able to provide a new registration code or license file.
Hence the only working way is to restore your Windows 10 system using System Restore (from a time preceding the license issue).
So make sure you Create a Restore Point regularly, do not depend on System Restore to automatically take snap shots. As these are usually after a program or system update has been installed - and may be quite old.
Please note: Any programs, drivers and even data that was added/created/deleted since the date of your System Restore point may be affected
The 'Unofficial' Tacx Forum
The unofficial Tacx discussion forum - no affiliation to Tacx