Just curious if this will cause any issues I am not aware of concerning retrieval and overall performance. I seem to be running three ok so far. The 4th drive would only be for samples.
 
 

The answer is basically no.

When you have a bunch of hard drives, sometimes you have a little latency while it wakes up the sleeping drive. This is less with solid state drives.

Basically because you have more drives it is more likely that the one with the stuff you need is sleeping. Spin up time is usually a second our two.

Your specs show you already have five 
 

Only thing to make sure of is that you have enough power capacity in the power supply (which you likely do.)

I've got 10 internal drives in my main studio DAW.
You'll be just fine...  

I just added another SSD for samples as well, now using 7 internals w/o any issues. ( on my Platinum Studio built by Jim)

My Mac Pro runs Win10 primarily. All 6 native SATA2 ports are occupied with drives (3 SSD, 3 HDD) and there is also an internal 3-way hardware RAID-0 that is the 'scratch' drive.  Furthermore, there is a RAID-5 external made of 4 HDD's that runs as a media drive for all audio/video projects.  CbB is happy in this environment with no issues. 
 
The external backup is managed in a pair of 4-drive USB 3.0 towers.  There are pictures of this in articles on my website about Homemade Hybrid Disk Drives.  I turned on everything at once during testing, and despite the madness of 19 drive letters in use (there are some logical partitions) CbB never flinched. 
 
My power profile keeps everything on, and says 'no' to any power saving spin-down options. This was initially for ensuring the SSD's didn't do anything rash, but I think other troubles have been concurrently prevented through this policy. By the way most of my 3.5 inch drives are Hitachi (HGST) 7K3000 and 7K2000's that each draw about 10-watts peak in 4k random reads, and around 7-watts at idle. 
 
 
 

+1 to the above, the only thing to check with your MB is what happens if you go to x4 mode on an NVMe M.2 drive. On mine, that takes up (and takes out) the SATA 5/6 connections, but that NVMe makes one rather nice sample drive.
 
Oh, the other one that was sort of funny when I did this... BIOS and Windows both recognize the drive, but Windows Explorer did not... had to go into Disk Management to bring it online after installation.

FWIW, If your motherboard has two M.2 NVMe slots, using the second slot at x4 (4 PCIe lanes) results in SATA ports 5/6 being disabled.
 
You're usually good-to-go using the first M.2 slot at full (x4) speed.
 
BTW, You can get around this by putting a second M.2 NVMe drive on a PCIe host card.
Place that host card in a full-length slot... and it'll give you the full speed of the drive.  

BobF
Your specs show you already have five 
 

I have more than that if I count all of my outboard drives. I have three internals . I never liked outboard drives very much. Easy to install yes. They use up valuable usb ports though. My MOBO only has one usb 3 port. The rest are 2.0. They decided to put those all on the back except for one. I would almost rather eat tarantulas than attempt to crawl to the back of my studio desk. I could get lost back there for months. I have a port hub but you probably know how they can muck things up occasionally. The ports are becoming more and more valuable. Waves licenses, my mouse transmitter,etc. I might be getting a dongle. Growing tired of cloud authorization with my McDSP plugins.
The cords they give you with outboard drives are like 4" long. Can't go very far with that length.
 
Thanks to Jim and others for your helpful comments. I noticed one of my drives is giving me a red volume indicator. I believe I have maybe 38gigs left on it. With platter drives it was never a good idea to go within a mb of filling the drive up, or so I have been told. Not sure about SSD. Do SSD's begin to loose capability when filled to almost full? If not is the red indicator intended for HDD's?