It's actually not that straightforward to plug in these consumer cards as 4x setup. We spent weeks researching how to achieve up to 7x RTX 3090 setup in a single rig. Could write up our method if anyone is interested.
We tried bunch of setups with Threadrippers and EPYC, at the end settled for the ROMED8-2T which is a monster motherboard.
The advantage of EPYC is that because it's so common, we can find used cheaper ones on ebay. They are a bit slower I believe, but we can deal with that by using Nvidia's DALI and decoding images on the GPU rather than CPU.
We're doing photogrammetry not machine learning, running some blackbox software that scaled best with clock speed so threadrippers were the most efficient option.
I put them in 4U supermicro boxes with a noctua cooler with 2 9000RPM delta fans attached to it with rip ties.
They must scream and push air like hell
Maybe some day I'll use it myself to train my plastic surgery outcome estimation visualization GAN or diffusion model if I can figure out how to fine-tune one.
we mostly gave up and just got barebones machines since the cabling situation becomes pretty tricky, and the barebones total cost is low relative to the GPUs anyways.
The issue we had with barebones was cost and cooling, we use 30$ racks from Target and hang the GPUs with metal zip ties and a box fan from below, so they get lots of air and we don't break the bank and can easily roll them around.
It's not exactly a "clean" one, like a proper 2u/4u chassis and server grade GPUs but it does the job for 70-90% cheaper.