The end of October PS4 was probably at 13.5 to 14M, by the of the year it's not unreasonable for it to hit 20M with the way sales have been going.
We've seen what kind of an effect games like Destiny have had on the PS4 numbers throughout this year, even TLOU remastered caused the numbers to sky rocket, next year PS4 has Uncharted, Ratchet & Clank, lots of other exclusives announced, two new IPs in The Order and Bloodborne, there's also the possibility of Gran Turismo coming out by the end of next year. No doubt there will be a price cut. We have big 3rd party multiplats like Arkham Knight, Witcher 3, The Division, MGS 5:PP, SW Battlefront amongst others which will all likely be hugely favored on PS4.
With a decent price cut pretty much inevitable as Sony reduces the costs of the system and with being supported by an even bigger line-up of games.
I wouldn't be surprised if PS4 sells well over 20M between Jan 1st and December 31st next year just off of the back of games mentioned above.
There's also things like Morpheus, a global PS Now subscription service and other potentially unannounced big exclusives that could push numbers up even higher.
