Ok, I must be doing things all wrong. I did some ISO tests and at 1080 24p - ISO 200 was horrible and grainy, and progressively got worse as ISO increased. When shooting in 4K, it wasn't bad up to 800. Also, I noticed 1080 seems cropped vs 4k. (after uploading to Vimeo, noise doesn't look AS bad.)
https://vimeo.com/213012841
https://vimeo.com/213010066