It's managed to sell approx 10 million* with a relatively small library lacking any killer exclusives. But the holiday's are looking pretty damn good with major games like Destiny coming out (there's even a Destiny bundle that comes with white ps4). Being that people are starting to make the switch to next gen more and more, will we see PS4 reach 20 million by year's end? Will it even come close? 20 million is pretty nuts, but I can see it coming close. PS4 is the obvious choice for multiplats and there's going to be a ton in Oct-Nov. We're already seeing less and less people go with last gen versions and I'm sure there will be some sweet deals to get people to make the switch. After all, $400 is a still a very good price for PS4.
** EDIT - Sony has confirmed 10 mill sold! http://www.theverge.com/2014/8/12/5995275/sony-has-sold-10-million-playstation-4s