It is not worth it. They sell a seperate LCD touch screen already that works for Hero 3 and 3+ so I see no reason why it will not work for hero 4 black.
http://www.thesource.ca/estore/prod...D-4u-AGIMoB4tgI9WuvzqAom5rCLKKJ4jMaAhy98P8HAQ
Only problem with this is it will make your gopro a bit more bulky but they also make the bigger waterproof housing to handle the attachements like LCD or extra battery.
Regardless of the touch LCD you will not need it since the camera covers approx 170 deg FOV. You quickly learn how to palce it and is forgiving a lot becuase it captures so much in a frame. If you are doing the tripod or timelapse just use your app via wifi to set up frame.
I think that 120 fps at 1080p is huge benifit and outweighs the benifits of a built in lcd touch. Plus the 4k is awesome as you can pan, edit video into 1080p and do so much to the 4k frame without loosing quality past the 1080p plus it future proofs your clips.