I wholeheartedly agree with this idea. Transferring from FFR and Stepmania got me used to a 4K play style, and although I am an extremely experienced 4k player, I still struggle with 5, 6, 7, and 8K.
I think that each key mode should be ranked individually. You cannot compare a 7K player to a 4K player and vice-versa. Each mode requires a different focus and trying to amalgamate them into one ranking system makes no sense. Currently, with the small amount of 4K ranked maps (HD/MX), getting to the top of the PP leaderboards is nigh impossible. You would have to play 7K maps in combination with 4K maps to amass a high enough PP count to get anywhere. For me at least, it just feels like there is a relative limit to how far you can get in 4K ranked.