the way it works on current consoles is that the game is put to lower settings automatically when 4 people are playing split screen. this is because split screen is the same as running the game 4 times simultaneously, and its no different with 4 tablets (except u would be running it 5 times instead of 4). anyway, this would be handy for games like super smash bros and mario kart and such. i think its quite necessary for those games so that everyone can have an equal experience, and the games that will use it are mostly going to be games that dont take much processing power, eg mario kart etc
But in splitscreen the console renders the same amount of pixels as with single player. In Wii u's case, the controllers adds up more pixels to render.