Infamous tech talk http://www.dualshockers.com/2014/04/02/how-infamous-second-son-used-the-ps4s-8-4-5-gb-of-ram-cpu-and-gpu-compute-to-make-our-jaws-drop/
This follows in the footsteps of The Order 1886 and Killzone Shadowfall, both of which developers revealed to use 4.5GB RAM. Those are the only games we have gotten RAM usage figures for, so all the PS4 games we know of use 4.5GB. It's likely that is the cap currently.
Xbox One has 5GB available for games.