External HDD not detected by Finder

If you are seeing "disk4s2" it either means the volume is encrypted & locked, or it means the volume is damaged or corrupted (perhaps due to a failing drive). If you use Disk Utility to attempt to mount the volume, it should prompt you to unlock it if the volume is encrypted. If you successfully enter the password and nothing else happens (no errors other than perhaps a mounting error), then most likely the encrypted volume was unlocked, but is damaged & won't mount.

Did you try running First Aid on "disk4s2" to see if there are any errors (assuming the volume is unlocked since TM backups are encrypted automatically now)? I would even recommend clicking "View" within Disk Utility and selecting "Show All Devices" so that the hidden Container appears on the left pane of Disk Utility. Run First Aid on the hidden Container of the TM drive...it will scan the Container and all APFS volumes within that Container.

Did you select "hard disks" in Finder's Settings?

Is Time Machine working? If it's not mounting then I doubt it. You might need to reformat the disk. Disks that are operating in a state of failure will take a long time to mount, or won't mount at all.