How are you doing video capture? NDI, HDMI capture card and USB? straight USB webcams?
(beware chipset and USB Root Hub limitations)
Will you have a video switcher (Atem, etc) such that PC only handling 1 video stream at a time, or PC handling all 4 video streams simultaneously. If PC processing all 4 4K streams, then the specifics are the driver and its interface will have a significant impact. Which video protocol? And will you be able to set up for GPU decode of those streams?
AMD CPU has more powerful CPUs and for equivalent CPU power use less electricity, but are harder to come by.
And Video decode/re-encode processing is only part part of workflow. Part of CPU equation depends on what else the CPU is doing, and which settings/effects/filters are used which may drive CPU, RAM, or GPU utilization
