I went with Pacroban 8K HDMI 2.1 cables for future proofing purposes in my setup from this post:
https://www.reddit.com/r/kindafunny/comments/ik9l9y/spent_the_last_month_building_my_gaming_setup_its/

They are not super expensive and have been recommended by people like Austin Evans.

Here's a link on Amazon (currently rated 4.7/5 with over 1,000 reviews):

https://www.amazon.com/gp/product/B07VDBCKZV/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1

If it's more than 10ft, you'll want a fiber optic HDMI 2.1 cable that has the chip in the connections so that you don't get signal/quality loss. This is when it actually does get quite expensive (for now) and is worth spending the big amount.

I am able to get 4k 120hz working in HDMI 3 and HDMI 4. An issue I'm having, is in my NVIDIA Control Panel, under PC Resolution, I can only select 3840x2160 at 120hz or 100hz, it can only select 4:2:0, and only 8bit output color.

Does anyone have tips?

I have a pacroban 8k cable:

https://www.amazon.com/Pacroban-Braided-Supports-48Gbps-Dynamic/dp/B07VDBCKZV/ref=sxts_sxwds-bia-wc-drs-ajax1_0?crid=2YYYOS3AVCXAT&cv_ct_cx=pacroban+8k+hdmi+2.1+cable&dchild=1&keywords=pacroban+8k+hdmi+2.1+cable&pd_rd_i=B07VDBCKZV&pd_rd_r=8a78539a-679f-4f5a-a291-ede5edad9194&pd_rd_w=QA8UC&pd_rd_wg=M6U5l&pf_rd_p=2af54486-024b-43c2-a014-77f8e1ab2526&pf_rd_r=AQE2QKJ1W0FH0C4HHTZZ&psc=1&qid=1603242544&sprefix=pacroban%2Caps%2C197&sr=1-1-25b07e09-600a-4f0d-816e-b06387f8bcf1