A different version of PostgreSQL won't be able to read your old database. If you have all of your hand histories then you can import your hands to the new database, or if you have a Backup then you can restore that. To be able to connect to the old database you would need to reinstall the same version of PostgreSQL that you previously had.
If you're going to reimport you may want to install our recommended version 9.3; 9.5 is pretty new and we haven't yet confirmed that there are no issues affecting PT4.