Anyone know of a nice 4x disk 'pack' with a fan and single Thunderbolt connection that allows me to use these disks as though internal (and NOT a raid array)?
Sure, here's one:
https://eshop.macsales.com/item/OWC/TB3IVKIT000/
What version Thunderbolt does that 2013 Mac Pro have? I'm guessing it will be Thunderbolt 2 (or maybe even TB1). If so, you would have to use Apple's pricey
Thunderbolt 3 to Thunderbolt 2 Adapter to use a "modern" TB3 device. Looking for a TB3 device will give you more choices and would work with any of the newer Macs with TB3 (or future TB4).
Alternatively, you can still find some Thunderbolt 2 devices (check ebay). Any future TB3 Mac could use the older TB2 device with the aforementioned adapter.
One thing to keep in mind while shopping is that even if the device has built-in (hardware) RAID, you can usually set it to "JBOD" to turn off the RAID feature. Each disk would then appear as a separate drive to the computer (verify that feature before you buy, though). So if you see a good deal on a "RAID" enclosure, you still might consider it.
I'm partial to Akitio thunderbolt 2 enclosures because I have three and they've been rock-solid stable for years -- never a dropped connection. However, a quick search didn't turn any for sale.