The format of the 4D datafile has not changed between v15 to v17 and as such, as long as a valid structure file for the datafile exists for a version the datafile can be moved and opened in any of the versions regardless of the last version it was opened with.
4Dv17 R5 introduces a new excryption feature that allows specific tables to be encryted. When a datafile is encryted, the data is view as null and are displayed as such with strings display "" and numbers display 0s.
In versions with the encryption feature, if the record is attempted to be modified without the encrytion key an error will occur and state that there is no matching key and new records are prevented from being added.
If an encryted datafile is moved back to an older version, v17R4 and below, the encrypted data will still be hidden. However, when trying to do the same on a version without the proper error codes and messages implemented, an error will occur on the header or index.
If the data is decrypted before moving to an older version, everything will still be fine. Thus if the new encryption feature is used, make sure that the data is decrypted should the scenario of reverting to an older version be needed.