Which cable for Time Machine SSD on iMac M4?

USB followed with a letter is a connector.

USB with a number is a protocol version, and different connectors can be used.

Thunderbolt connections are not usually relevant here, as most folks are not buying Thunderbolt-capable mass storage. Not unless you’re (for instance) buying a storage array.

USB-C connectors can support multiple protocols, including USB, USB Power Delivery, Thunderbolt, and other connections. Here, you want to use (buy) USB, and your Mac has USB-C connectors that support both Thunderbolt and USB4.

Okay, with all that written…

You’ll want a hard disk or SSD with a USB-C connector for the computer, though the older USB-A connectors can be made to work with an adapter.

A storage device with USB4 or one of the USB 3.x variants will work. You don’t want to buy a storage device with USB 2.0 or USB 1.1.

The documentation and the packaging will usually (always?) indicate USB-C or USB-A or whatever the particular device was built to connect with, and the package will usually include the cable necessary.

Most mid-grade and higher (recent) storage gear will have a USB 3.x or USB4 connection with USB-C connection on both ends of the cable, using USB-C on the computer snd USB-C on the device.

A hard disk will be cheaper and/or with larger capacity, and a somewhat slower Time Machine backup will not bother things with an iMac. An SSD can make backups faster, but at a higher price for the capacity.

The storage should be roughly 2.5 to 3 times the total storage capacity of the Mac internal storage and any configured external storage also needing backups, or more.

If you have multiple Macs around, a NAS can be a reasonable choice; network-attached storage.

Any drive which connects through Thunderbolt 3+ or USB-C, as well as earlier Thunderbolt ports and USB with an adapter.

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