I feel like I'm going to spend a lot of unneeded time telling people this, so until it sticks... well, I'm going to stick it.
Do your experiments, very much encouraged, but you'll see that 400ISO, even underexposed, is really what this camera wants. 800 ISO isn't BAD at all, and to me it even offers an interesting look that's slightly different in ProRes.
But 400 ISO is the 4K camera. It loves it, and will return many good images to you in exchange for it, even if it's being starved for light.
Let's get out ahead of the confusion that a higher ISO means better lowlight performance. =D It took me a bit to get it, as well.
Any other B4K shooting tips are welcome.
Although I shoot 400ISO, I treat the camera as 640 ISO, or rate it as that when approaching light. That means I'm underexposing images a bit. To me, I feel like this gives you the fattest image to work with where DR is concerned. Noise penalty is not great at all and color remains rich.