I built a 4 GPU system for 3D rendering. I had done a lot of research and learned that blower cards were the way to go for something like this. I have 2x GTX 2080s and 2x GTX 1080s, plugged into an X399 motherboard running a TR 1950X, and man did this this thing get hot. Even with 9 fans in the case, the GPUs were still hitting over 90c and throttling. My only solution was to leave the side panel off and point a desk fan at the cards. Obviously, that wasn't going to fly. The last image above is what the layout was in the old case: Lian Li PC-O11.
At this point I had 2 options, water cool all of the cards, or try to space them out. A new case was definitely going to be cheaper than a loop, so I purchased the biggest case I could find: the Cooler Master Cosmos II Ultra Tower. This case is a monster with a 200m intake fan at the front and 10 expansion bays at the back. But even with 10 expansion slots, the spacing on the motherboard would only allow me to insert 3 GPUs with space between them. The 4th GPU had to go somewhere else.
There happened to be just enough room in the basement of the Cosmos if you removed the HDD caddies. I made a wood cradle for the GPU to keep it elevated off the ground, and used a 600mm PCIe Riser cable to route it to the motherboard. And wouldn't you know it, this crazy layout freaking WORKS!!!
All of the GPUs now run in the mid 70s under full load, and there's no noticeable performance loss from the riser cables. I'm so happy I was able to figure this out, and since I couldn't find much on the web about 4 GPU builds, I figured I would share it here for you all to see. Thanks for checking it out.
Some things to note:
I'm using a 360 CLC, but the Cosmos does not support 360 (the old case did). I was able to bend some pins out of the way to make the 360 fit, but I would recommend a 280 instead.
I'm using Thermaltakes Premium PCIe Riser cables. There's a 600mm one for the GPU in the basement, and a 300mm one for the GPU that's just below the motherboard.
When booting up the system for the first time, Windows only detected 3 GPUs. This drove me absolutely crazy. Thanks to a kind soul on reddit, I was able to fix it by going into the Device Manager and updating the driver on the VGA controller.