No BIOS needed for Jaguar at all?
No BIOS is required for this console:
Used Emulator/Core (libretro-virtualjaguar) is not ready (yet?) to work properly on Pi 4? And/or it maybe will never work properly on Pi 4, because the complex original Jaguar hardware makes emulation quite difficult and/or needs more power for emulation than the Pi 4 provides.
It's something between these two options, the emulator currently requires a lot of hardware as it is not optimized enough, and maybe Rpi4 will never be able to emulate this console properly.