Can I use a Western Digital My Passport 4TB to store my photos from my iMac? I do not want to pay rent to iCloud for storage and am told I have a large file that a rented option is necessary.

Did a Genius from the "Genius bar" tell you that? The only Genius to be found there is the one who figured how to get Apple users to pay for something they don't really need. A USB drive will store any file for as long as it fits, flash drives may be pickier due to file system restrictions to maximum file size. But that should not be an issue with large capacity drives. If the Apple store was an actual Apple the so-called geniuses would be the worms eating at its core.

Can't help with Apple..