I typically look for 1080p X265 encodes around 2-4 mbps to save disk space. I will download higher bitrates for anything with a lot of film grain since it will get very blocky at lower bitrates.
I can't tell much difference between 1080p and 4K unless I'm very close to a large screen. Also, most 4K files are HDR and I don't have anything that supports HDR.