There are currently 9 free PS4 games available to add to your PSN account. Only available from March 25 8pm PDT to April 22 8pm PDT. No playstation plus required. Just log into PSN and add to your library:
https://store.playstation.com/en-us/category/9779f12f-50c8-4c07-8d20-47e21293b6c6
Ratchet & Clank is also still available for free until March 31 08:00PM PDT. Same as above, login to PSN and add to your library:
https://www.playstation.com/en-us/games/ratchet-and-clank/
Horizon Zero Dawn complete edition will be free from April 19 until May 14:
https://www.playstation.com/en-us/play-at-home/