God damn, I suck for this. Only going to list the ones I've QUAD-DIPPED (or more!) for. Obviously, for some of these, the game was included in a bundle/collection for more recent systems.
I also don't own most of these duplicate copies anymore. For the physical ones, they've almost all long-since gone, been sold, traded, etc. Just counting them as purchases, of course.
4x: Final Fantasy XII: PS2 (NA), PS2 ZJS (JPN), PS4, Switch
4x: Final Fantasy IV: SNES, PS1, GBA, DS -- and I don't even like this game. lol
4x: Final Fantasy V: PS1, GBA, iOS, Android
4x: Mega Mans 3 and 4: NES, PS2, PS4, Switch
4x: Castlevania - Symphony of the Night: PS1, PS3, PSP, PS4
5x: Resident Evil 4: GC, Wii, PC/Steam, PS3, PS4
5x: Final Fantasy IX: PS1, PS3, PC/Steam, iOS, Android
5x: Final Fantasy VII: PS1, PS3, iOS, PS4, and soon, the PS4 Remake!
7x: Final Fantasy VI: SNES, PS1, GBA, Wii VC, Android, iOS, Android again (this time on my own account -- first one was the BF's, lol)