Personally, I'd rather have good, solid 2k then compromised 4k.
You can get WAY better HD from a 4k cam because you can downsample and get 4:4:4 and get rid of a lot of compression artifacts. You will essentially have a 100mb/s HD camera at 4:4:4. Think about it.
Same thing as you can get WAY better SD from HD cams.