There is unlikely to be a demo for God of War on PS4 because it takes a way a lot of development time according one of the game’s developers.
God of War PS4’s producer, Cory Balrog tweeted: “hard at work on the game, no demo. game is be better the fewer demos we do. Demos pull a month or two away from final game.”
The game is due out sometime next year exclusively for the PS4. They probably want to release the game on time without delays which is why no demo is being planned.