Recommendation for a backup external iMac 24” M4 running Sequoia

Most any USB hard disk drive or SSD will work. Best to find one that is self powered, that is a desktop class drive that come with it's own ac power adapter. Laptop class drives usually work well, however. If the drive comes with drive utilities from the manufacturer, don't bother installing those on the Mac. Those utilities are not needed on the Mac and often cause issues.

Many veterans here in the Communities will recommend drives from OWC/macales.

Make sure the drive is formatted properly for the Mac. You can use Disk Utility to format it.

Apple provides guidance here:

Back up your Mac with Time Machine - Apple Support

Types of disks you can use with Time Machine on Mac - Apple Support

+1 on D. I. Johnson's recommendation of desktop-class (3.5-inch) drives that are self-powered. For every report here of problems with a desktop-class self-powered drive, we probably see 30 involving cheap 2.5-inch "mobile" or "portable" class drives that must scrounge all power from the computer.

Reliability in backups is my prime driver in my choice of backup configuration. My income-producing iMac 5k has two of the OWC Mercury Elite Pro drives the others mention. One makes Time Machine backups and the other makes a bootable copy of my boot volume before I wake up each morning. We have five of those OWC drives backing up two iMacs and one Macbook Pro.

An SSD is overkill for Time Machine backups. Once the initial backup is complete, a 7200-rpm SATA6 mechanical hard drive makes subsequent incremental hourly backups quickly and unobtrusively.

When you ask yourself, "How much is my data worth?," the extra cost of quality external drives is quite bearable.

Hard to beat the OWC Mercury Elite Pro, however you will need a simple USB to USB C adapter which you can get on Amazon for <$10

I can recommend the Toshibas - excellent value for money https://storage.toshiba.com/consumer-hdd.