USB-C to firewire 400

If the FW400 device has its own power suply, it should work through these adapters:

Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter - Apple

Apple Thunderbolt to FireWire Adapter - Apple

https://www.elago.com/firewire/firewire-400-to-800-adapter-white

Otherwise you would need the OWC 13 port Thunderbolt 3 dock.

https://eshop.macsales.com/item/OWC/TB3DK13PSG/

They are no longer being manufactured, but some are still available.

You will still need the FW800 to FW 400 adapter.

https://www.elago.com/firewire/firewire-400-to-800-adapter-white

Sorry, can't be done. I've looked into this more than a few times. The pinouts, wiring and the FW interface in general is incompatible with any flavor of USB.

If you try to look up converters, you can easily get a lot of sites that say "firewire to usb converters and cables". But when you go to any of them, there's no such item available.

Firewire 400 is severely out dated.

I would expect to see no USBC to Firewire400.

update your backup hardware.

I would not recommend stacking adapters.

Apple Thunderbolt to FireWire Adapter - Apple