When a binary structure (.4DB) of a database is upgraded to the next 4D version, it can no longer be opened in previous 4D versions.

However, project mode structures (.4Project) can be opened in previous 4D versions without an issue. Currently, it is possible for project mode structures to be opened across v17R5 to v19. The oldest version being v17R5 becuase that was when project mode was introduced. This is because project structures do not need to be converted like a binary structure.

In case you have a binary structure and you want to open the database in a previous version for any reason, just export the structure to a project and open the project structure in the previous version.