https://github.com/Arc-Compute/LibVF.IO/tree/master/ plus https://github.com/gnif/LookingGlass works pretty well. If you use an Intel GPU, particularly one of their new Arc dedicated GPUs, it supports the functionality on the consumer grade hardware without any trickery and you just need Looking Glass to map the outputs.

If you really want multiple GPUs though you can also use a normal 1-in-4-out type PCIe switch and save a lot of cost on Thunderbolt components in-between. Low bandwidth ones are particularly dirt cheap due to crypto mining volume. Keep an eye out for ACS support or you may have to use an ACS override patch though.