Onimusha: Tactics is available on PC, but I think you might be referring to Onimusha: Warcry, which was released on PC in 2018. However, I found that Onimusha 4 doesn't exist, but a game called Onimusha: Soul Genma was released on the PlayStation 2, and another game called Onimusha: Tactics was released on the Game Boy Advance and later ported to the Wii U Virtual Console and New Nintendo 3DS Virtual Console.
As for downloading Onimusha 4 on PC, I couldn't find any information on a game with that title being available on PC. However, if you're interested in playing Onimusha: Warcry on PC, here's how you can download it: onimusha 4 pc download
If you have typed "Onimusha 4 PC download" into a search engine, you are likely feeling the frustration that plagues this fandom. Why can’t you find it on Steam? Why aren’t there official links? And most importantly, how can you play this forgotten PS2 masterpiece on your gaming PC right now? Onimusha: Tactics is available on PC, but I
Assuming you want to preserve your physical disc or are willing to create a backup, here is the definitive guide to playing Onimusha 4 on PC. Wait – Capcom has registered trademarks for Onimusha
: For many, the only viable way to play the game on a modern computer is through PlayStation 2 emulators like PCSX2. This allows for increased resolutions and stable frame rates, but it requires users to possess a digital "ISO" of the game.
: This is the primary method for running the game's ISO files on modern hardware. HD Remaster Project
You will need a legal copy of the game's ISO file and your own PS2 BIOS.