4 Silver is the first to have built-in screen but 4K support tops out at 15fps (like 3+ Black)
4 Black does not have screen but has 30fps 4K, and overall higher framerate support in other resolutions.
I'm guessing the 4 Silver is aimed at the prosumer extreme-sports type who can benefit from the all-in-ine package, whereas the 4 Black is aimed more at pro level action, B-roll and crash cams.
I have the 3+ Black and I use 2.7K/30fps a lot (downsampled to 1080P in post) and find that my PC struggles enough to process that, so I would be happy with the 4 Silver without usable 4K.
4K support will be important in the future, but by the time its a 'must-have' feature for amateurs I'm sure we'll be on to the GoPro 7.