SanDisk portable SSD for backing up iMac M4

Yes, it will work, but when SSDs fail they all tend to fail catastrophically. That's one reason I prefer to use multitude of cheap, slow, disposable hard disk drives for TM backups. They are more rugged than one might think, and tend to exhibit symptoms of failure well before they become unreliable.

You can buy a 5 TB HDD for about the same price as a 1 TB SSD. But since 1 TB capacity is likely to be insufficient for TM, and one and only one backup drive doesn't comprise a robust backup strategy anyway, you can buy two 5 TB HDDs for less than the price of one 4 TB SSD.

Which is the better choice? Everyone's backup needs are different, so you decide.

It should work fine with one great big caveat... Some SanDisk portable SSD's were known to suffer catastrophic failures resulting in a complete loss of data. Do some research on the exact model you have before relying on it for anything critical.

Yes that drive will work,

see > Back up your Mac with Time Machine - Apple Support