Nikon 4000slide scanner

The Nikon Coolscan 4000ED has a Firewire-400 interface. You will need a Firewire cable with what's known as a 6-pin FW400 plug on the end that plugs into the scanner.

As for the end that plugs into your Mac, it depends on what ports your Mac has. I am guessing that your iMac only has Thunderbolt & USB ports. In that case you will need an Apple Thundebolt-Firewire Adapter. Note, however that the Firewire connector on this adapter is FW800 not FW400.

So, in the above case you will need a Firewire cable with a 6-pin FW400 connector on one end and a 9-pin FW800 connector on the other end ... like this one. Plus the Thunderbolt-Firewire adapter.

The scanner will work on a Mac with Big Sur, however the original NikonScan software will not. The software was discontinued many years ago. I suggest that you look into Vuescan instead.

FYI, regardless of its age, the Nikon Coolscan 4000 is still a respectable scanner and if it was cared for and is in good condition it will perform as well as it did when new. And although it has an (older) FW400 interface it is still possible to connect & use it with most current Macs & PCs with appropriate adapters. On recent Macs with Thunderbolt ports all you need is an Apple Thunderbolt-to-Firewire Adapter.

Apple did not "kill" support for Firewire with the advent of OS X. Apple continued to build Firewire ports into Macs up to about 2012. At that time Apple replaced the Firewire ports with Thunderbolt ports and released their Thunderbolt-to-Firewire Adapter. The operating systems (OS X and macOS) continue to support Firewire via the Thunderbolt ports.

Presumably you mean the CoolScan 4000? You do understand that thing's 20 years old, right? And you want to connect it to a computer released in 2014 (I'm guessing, as it's the earliest model which can run Big Sur) running an OS released last year? Just think about that for a second.

It requires a FireWire (IEEE1394) cable. The last version of a Mac operating system to support FireWire was System 9. Apple killed FireWire support with the advent of Mac OS X. The type of cable a device requires is dependent on the device, not your Mac or MacOS version.

In case you can't figure it out, what I'm saying is that what you want to do is impossible. If you have a need to scan slides/film, you'll need to buy something considerably less ancient!