I tried that; my PC was able to capture 8K but editing was very intensive and slow.
As would be expected. Just as real-time editing of 4K video takes a LOT more computational resources than 1080p video.. 8K takes that much more
It sounds like you might be more interested in separate 4K recordings per incoming video feed, and I'm not aware (and could easily be wrong) of a default OBS Studio config that does such.
If you are not making real-time compositing edits to such video inputs, OBS Studio may not be the right tool to use. There are other, tools, with lower resource demands (some command line based), to take a source input and record that input.