Hi. I have been mining for 2 years so I do have a lot of experience with it so maybe I can help....
Firstly I know it's a pain in the butt but I recommend you get rid of Windows 10 and try 8.1 or even better (if you can wrap your brain around it) use one of the Nvidia specific linux mining distros - nvOC is the first obvious choice
https://bitcointalksearch.org/topic/mining-os-nvoc-1854250 but it will require a steep learning curve if you have no Linux experience. It is worth it in terms of stability especially but like I said - if you don't know any Linux it will take a lot of work.
My next suggestion is much easier to implement - make sure that your Windows virtual memory is set to more than the total of vram on your GPUs. So in your case, I think the 1070ti has 8GB of vram? If so 4 x 8 = 32GB so maybe set your virtual memory to 40 GB. If you don't know what I mean....
right click on "my computer" icon -> properties -> advanced system settings -> in the "advanced" tab of system properties click the "settings" button in the "performance" box -> in performance options choose the advanced tab -> click the "change" button in the "virtual memory" box -> uncheck the "automatically manage ....etc" box -> click the "custom size" point and set the minimum and maximum to 40960 MB, or maybe minimum to 32768 and maximum to 49152. Then apply the changes and you may have to reboot. Also obviously the virtual memory has to be on a drive (or drives - you can have multiple virtual memory blocks) that has that much space spare on it. If you are only using one drive or partition for Windows it will obviously need to be on C: but will only work if C: has 40 or 50 GB of spare space. It's a very good idea to put the virtual memory on your fastest drive, so if you have a hard-drive and an SSD, put it on the SSD if you have the space.
The next thing that I will say is that 80% of the time when I have had problems, it has been because of a bad riser. Risers are made very cheaply in China and it seems they don't test them before shipping, because I have probably bought nearly twice as many risers as I have ever used because they either didn't work when I got them, or failed withing 1 or 2 months of using them. I prefer to use the USB 3 risers that are powered via a 6-pin PCIe 12V connector. You did say though, that you have hash-rate tested them all individually so the riser problem seems unlikely.
I'm not familiar with that particular motherboard, but I think your power supply is easily big enough, but I haven't used 1070ti before... I'm guessing they use more power than a regular 1070 but one of my rigs is 4 1070s
and a 1080 and that uses around 1000W. An important point there though is that I have set the powerlimit on those cards to 80% using MSI Afterburner. It's very inefficient mine equihash with your GPUs at 100% powerlimit unless you have free or very cheap electricity. Are you using any overclocking/underclocking software like Afterburner or Nvidia Inspector or whatever? I'm guessing not since it would be obvious that you should try to start mining at factory clock settings and under/overclock later when you are stable, right?
What software are you mining with? Have you tried both EWBF and DSTM for equihash? Have you tried mining any other algorithms like ETH, Lyra2rev2, neoscrypt, etc?
Anyway - I think the most likely one of my suggestions is your virtual memory needs to be much higher, especially considering you have only 8GB RAM. I know RAM is expensive, and everyone says use as little as possible, but I have always found that you get much more stability on a rig with 4 or more GPUs when you have
at least 8 GB RAM. Most of my rigs have 12 GB on them, one 8 GB stick and one 4 GB stick. It doesn't matter that it's not dual channel - it's the extra amount that seems to help. Unfortunately since you are running Windows 10, there is a lot of crap running in the background tracking everything you do so it's using a lot of RAM.
Good luck, I hope this helps and please ask me specific questions if you have any.