One Piece: Pirate Warriors 4 is a massive hit in the musou genre, it was never officially released for the PlayStation Portable (PSP). Any "PSP ISO" files you find online claiming to be this specific game are likely fan-made mods or deceptive files.
The confusion is understandable because One Piece: Pirate Warriors 2 receive a PlayStation Vita release in Japan and Asia (with English subtitles in some versions). The Vita was the PSP’s direct successor. Many casual gamers blur the lines between "PSP" and "Vita," assuming that if a game existed on one Sony handheld, it likely existed on the previous one.