NAS means Network Attached Storage, so the first question for you is: Is there a requirement that this backup system has to be on the network?
If there is no actual need for it to be on a network, you should strongly consider DAS (Direct Attached Storage). All that means is to have a drive enclosure connected to your computer using USB 3 or Thunderbolt, instead of Ethernet networking.
Why is this important? Speed.
If you use an NAS, the top speed is limited by the type of networking you are using. If you will connect to the NAS using Gigabit Ethernet, it can’t go any faster than 1 Gb (gigabit) per second. But even that theoretical maximum is slower than most good hard drives. (Maybe not a big deal for backups, but Lightroom users typically need to move many GB (gigabytes) of photos and videos in a timely manner.) You could connect using 10 Gb/s Ethernet, but this is not common so it costs more, and if your computer’s Ethernet port and your router don’t already support 10 Gb/s they will all have to be upgraded.
A DAS connected using USB 3.1 (
such as this model) is fast enough to not bottleneck any hard drive, and is also faster than cheaper SSDs. That's because USB 3.1 is either 5 Gb/s or 10 Gb/s depending on your computer — both many times faster than Gigabit Ethernet, but no extra cost because your computer already has what you need.
An NAS can be awesome if you need it to do other jobs for you. Do you need an NAS because it’s an always-on networked server that can back up multiple computers, serve music and movies to your TV and mobile devices, be a (non-backup) file server for documents the whole family or business needs to get to, record video from your surveillance cameras, function as a private cloud server when you’re mobile, etc? Then you want an NAS. But if all you need is an external USB enclosure with a 4TB drive inside purely for backups, then that’s all you need to pay for. That and reliable backup software.