I would also consider two RAID1 mirror sets, instead of RAID10 or RAID5.
And would also consider JBOD.
Here are my reasons:
RAID1 is more reliable then RAID10, because:
a problem with single disk affects only the corresponding mirror set, and not the whole array.
Same goes for transient errors such as disk rebuild - affects only the related array.
In case of controller/device error (which is quite expected on low cost nas devices), it is easier and safer to gain access to data from mirror set then RAID10.
RAID1 gives you same disk capacity as RAID10, it is just divided to 2 distinct repositories instead of single one.
Actually it gives you a bit less, because you have to manage allocation and free space on each mirror set, but with good planning you get similar backup capacity.
RAID1 gives you same performance as RAID10 (because you can target different jobs to each mirror set and run them in parallel if you wish to).
You can also consider JBOD configuration - each disk by it own.
Targeting different jobs to each disk.
VM group1 daily backup to disk1
VM group 2 daily backup to disk2
VM group 1 weekly backup (or backup copy job) to disk 3
VM group 2 weekly backup/copy to disk 4
RAID5 gives you large capacity in single repository 6TB, but has write penalty and is more prone to errors especially on large commodity sata drives. (double disk failure, rebuild problems and time, etc).
RAID10 gives you less capacity 4TB, better failure tolerance, in single volume.
RAID1*2 gives you similar capacity to RAID10 but in 2 volumes, similar performance to RAID10, a bit better error tolerance.
JBOD*4 - gives you most capacity 8TB divided to 4 independent volumes. Failure tolerance achieved by storing backups on more then a single device.
Expected performance similar (or better) then other options, as you can divide workload between jobs and disks, thus running several write intensive operations on them can provide better performance in total.
I have no bottom line single answer, just suggest to consider all options and see which best fits your needs and preferences.
I do have an important note - regardless how you plan and configure the NAS, do not use it as the single and only repository for your backups. Make sure you have at least 1 other copy on other device (USB3 disk, other NAS, internal SATA drive of other PC, tape, whatever). 1 of the copies should be offsite (manually taking detachable media with you, and/or offsite via network).
Read more about 3-2-1 rule or whatever you wish to call it, just don't put all your backups on single device.