Since Disk Utility First Aid failed to repair your WD My Passport 4TB external drive, your external drive should be damaged severely and can't be mounted normally on Mac. As you can't see the drive in Mac Finder, you can't access the 600GB files in a normal way.
Luckily, you'll still have a chance to retrieve the 600GB files from the WD My Passport 4TB external drive with the help of professional data recovery software. That is iBoysoft Data Recovery for Mac. This powerful app is capable of recovering files from corrupted external hard drives, SSD, SD cards, and USB drives on Mac.
Downloading and launching this easy-to-use macOS program, use it to scan the corrupted/damaged WD external drive and search for any lost files, then you can preview the scanned results and recover all the files you want.
After successfully recovering data from the WD external drive, you can format the external drive on Mac. Reconnecting the external drive again after reformatting, it should be mounted in the Mac Disk Utility and Mac Finder correctly.