The official Power Requirements of the Pi 4B is 5v, 3.0A - I.e. a total of 15W maximum power requirement from each of the Pis, so a 60W USB power supply, or larger, would be ideal. This is a maximum, as they acknowledge with:
We recommend a 2.5A (2500mA) power supply, from a reputable retailer, that will provide you with enough power to run your Raspberry Pi for most applications, including use of the 4 USB ports.
This month, I have built a 4 board Hadoop/Spark cluster following this guide.
I looked at the possibility of buying a PC Power Supply but for a 4-Pi cluster, that seemed like overkill and would generate a clunky solution.
To power it, I bought an Anker 63W 5-port USB Charger and 30cm USB to USB-C cables. There's no need for a USB-C hub and, as you point out, they don't exist in any case.
It's true, that only two of the sockets are rated at 3.0A, the others are described as 2.4A, but even with the draw from the fans that came as part of the cluster case (5V 0.2A) the cluster works perfectly well. I am running them headless and won't be having any external peripherals so I don't foresee any power issues with this setup.