If you have been using PostgreSQL v9.4 or later without any problems then you can continue to use a later version if you wish to do so but that does not necessarily mean there won't be a problem with that version further down the line. If you choose to change versions then PostgreSQL v9.3 won't be able to access your PostgreSQL v9.5 database so you would have to create a new database anyway and reimport your hands. PostgreSQL does change some functionality between major versions and while you could use the PokerTracker
backup/restore functions to transfer your data I would only recommend that if you were upgrading PostgreSQL and not going the other way. If you need to locate your hands by default PokerTracker 4 moves your processed hand histories to your \PokerTracker 4\Processed folder which you can access via the "File -> Open User Data Folder" option.
Guide: Processed FilesIf you do not have all the hands you need you can also
export hands from the existing database (and don't forget to export your notes if you want them via "Database -> Notes"). When you are ready you can then follow
this tutorial to reinstall PostgreSQL.