PS1 games on PS4: Which PlayStation 1 games can you play on PlayStation 4?
PlayStation 4 and PSVita Releases: PCSX-R port for the PS4 has been released allowing you to emulate PS1 games and ElevenMPV 2.1.0 released with many fixes! - Wololo.net
How To Play PS1 Games on PS4 - YouTube
How To Play PS1 PS2 PS3 Games On PlayStation 5 - PS5 Backwards Compatibility Guide. - YouTube
Why PS4 Doesn't Play PS1 or PS2 Discs, and Why Sony Probably Won't Ever Do it. - YouTube