I don't know about running 4 eGPUs but just for fun I have successfully ran 2 GTX1070 in 2 eGPU enclosure Daisy chain on a 2016 13" MBP running Sierra with a huge increase in performance. (Yes, knowing that the 13" MBP has no dedicated graphics onboard)
Please note: With the very little detail documentation publicly released regarding both Apple's hardware & software implementation and not taking the time to break down every variable and retesting the validity of my observation and speculation within my on personal experience. That said
With Apple's own proprietary implementation regarding both Thunderbolt 2 and Thundebolt 3. I personally believe that Apple's Hardware Acceleration integration control has a lot of effect pertaining to distrabuation of the PCIe bus lanes.
Testing the new Apple eGPU developer kit on the new 2017 15" MBP running High Sierra http://iphone.appleinsider.com/articles/17/06/06/apple-egpu-dev-kit-requires-external-monitor-nothing-unique-about-graphics-card-or-enclosure I've noticed that there was a huge performance differential with an AMD RX580 included with Apple's DevKit with taking the exact RX580 into a Akitio Thunder3 eGPU. And when combining both egpus together one the devkit would be recognizee. With alot of Researching (via YouTube) found this arrival from 9to5mac https://9to5mac.com/2017/04/19/akitio-node-gtx-1080-ti-gpu-macbook-pro-gaming-egpu/
Switching to a 13" 2013 MBP running osSierra, I took a couple of GTX1070 and with the Akitio eGPU first I loaded the script referenced from 9to5mac in the command line. Once everything was restated I then added Apples DevKit with the second GTX1070 Daisychained after the Akitio. As soon as my Mac recognizes the DevKit and restart (not plug n play) connected an external monitor daisychained after DefKik surprisingly had a signal.
Not having run any benchmarks, can only comment In regards to video rendering and an improvement in times not only to just adding a single 1070eGPU to a MBP will be boosted but an addition of a second 1070eGPU added a signicant amount of performance with a big decrease in overall lag.