If you don’t need to back-up the entirety of what you do, and want to stay on the cheaper end of things, even a used 2bay 213+ with the disks in JBOD will be fine to be honest.
My suggestion of considering RAID6 (5 isn’t even worth considering if 6 is available) stemmed from the fact you mentioned 4bays, but if you’re penny pinched and are willing to run regular backups you can start with a 2bay, use whatever disks you have, and drop a cheap 3TB external USB3 drive attached to it for backup and be done with it for a while.
Resale value is excellent on syn NAS, and you get to use what you have, get your feet wet, set your backups up and all, and upgrade later.
Running local drives + RAID6 NAS backup + second tier bacup on an external drive is some pretty damn serious safety for a home environment. Nothing wrong with it, ever; safety I mean, but it’s not necessary really if you have that second tier and you need to be budget conscious.
The removal of RAID6 or 10 from the picture also opens you up to cheaper NAS solutions.
I have a 213+ with two disks striped (mostly for the sake of it, there is very little perceivable difference from just JBODding them as they are 7.2k) and an external drive for second tier backup and am very happy with it.
What’s the amount of storage you need anyway? And what percentage of it needs backing up?