Although I think there will be a decline in PS4 sales, the majority of this prediction rests on the fact that I think the Wii U will do dismally.
Anyway, having read through all the posts in the thread, I still believe in this prediction! I think many of you are overestimating the appeal of a tablet style controller for the Wii U, and Europe (where sony is dominant) is going down the drain, and they will have less and less disposable income to spend on expensive luxury goods like the PS4.
My prediction threads:
Wii U will sell under 40m units (made on 14th September 2012)
PS Vita will sell under 20m units (made on 30th September 2012)
Wii U will sell under 7m in 2013 - I was right