Why would it be bad? FW800 is backward-compatible with FW400. There’s no reason not to connect a FW800 drive to a computer’s FW400 port.It could very well be 400 and I just assumed it was 800 (which would indeed be bad).
Once you get through this project, it’s probably time to speak with your higher-ups about updating your system. The latest iMac models are more than enough to hack away at 4k with speed and grace as long as you incerase the RAM (max it out if you can). Thunderbolt 3 has a much higher data transfer rates, which also means getting new working drives. But if you’re doing this stuff regularly for your job, you need to have updated tools that allow you to work without hitting these legacy-OS-related issues every time something breaks down.