My prediction for PS4 remains unchanged from past sales threads with a range of 115m-120m. This prediction includes the assumption of no price cuts from Sony, as I don't expect it this late.
PS3 sell-through (2013 - End of Life) = 14.9m
So if PS4 only matches the PS3 in this period (2020 - End of Life) it reaches 120.9m in sales. But PS4 has outperformed the PS3 in every other time period, I don't see why it would suddenly stop now.
In terms of shipments PS3 managed 7m after the launch of the PS4. PS4 is forecast to hit 110m by march, so if it can then ship another 3m in April-November they then only have to match the PS3 in post-successor shipments to hit 120m.
Basically anything less than 120m seems extremely unlikely. It would have to perform worse than the PS3 in this period.