There are other empty 4 bay solutions but they seem to cost more. Keep in mind that I'm thinking JBOD for this.
For JBOD, USB3 might be enough.
Config might be
Drive for Photos and Documents
Drive for Time Machine
Drive for Bootcamp
Drive for Less demanding Media editing
Generally I can't see more than any two drives in data use at once... unless I'm overlooking something like Time Machine running while access other drives on the same USB3 bus for example.
I'm open to alternate suggestions as well such as completely separate or multiple dual drive systems.
Risks, points of failure, bottlenecks, etc.?
Would it be better for Time Machine disk to be on a separate port?
Bootcamp might only need to be a partition on part of a larger drive but I see this as a potential problem depending on what the rest of the drive is being used for.
Odd thing is that the 2 bay version is now a 5 week wait as noted on OWC site.
4 bay version is 3 days but, as you note, Thunderbolt only.
I'm thinking of getting Qx2 just for JBOD use and maybe the 2 or 4 bay for editing.
If you're using 2 bay for editing what are you backing up to?
This is why I'm thinking separate devices. JBOD system would be to have media backed up.