I just upgraded the hard drive in my Playstation. It was remarkably easy following these very thorough directions. Sony designed the system to be easily upgraded, quite a surprise.
The hardware is super simple; slide off the cover, remove a single branded screw, and slide out the drive tray. Swap in the new drive (4 screws) and slide it back in, done. I replaced it with a 2TB hybrid drive. A full SSD would be faster but costs 5x as much. These Seagate SSHD drives have a 64GB SSD caching the 2TB platter and seem like a reasonable compromise.
The software is also surprisingly simple. Back up the PS4 to a spare external USB drive, swap in the blank new drive, then boot to recovery mode. You can then reinstall the firmware from a download Sony provides, restore your backup, and you’re done. It took all of 30 minutes of my time, plus 5 hours waiting for the backup and restores to copy.
My main reason for the upgrade was more space. The old units shipped with a 500GB drive which after overhead, etc allowed for about 6 games to be installed at once. Significant nuisance; I don’t want to download 30GB a second time for something I want to revisit.