PS4 is just catching up the ground. Wii catched up from everything since N64, but the Wii U unfortunately is loosing install base and Nintendo customers.
No wait, the Wii brought most of the casuals, and also the casuals went away after it.
The XBOX ONE could end up somehow outselling PS4 by the end of the gen. Nothing is for certain yet. (except perhaps, the positions of Vita, 3DS and Wii U.)