The PS4 will have one last push when Sony finally reduce the RRP cost of the base PS4 to under $200 (£200), this may or may not happen this year...but I fully expect it will happen early next year. Once this happens (alongside a price reduction for the Pro as well), then sales will sky rocket for a bit, before the PS5 is released.
Also, don't forget that there are still a decent chunk of console owners that still use the PS3, so when these people eventually upgrade when the cost is sub $200.
Even if the PS5 releases at the end of 2019....I'm sure we will still see solid PS4 sales for a good 2-3 years as we are only 4.5 years into it's cycle...and I fully expect it to have a longer cycle than the PS3.
Even after all that..I will go with a conservative prediction of 118m.
Prediction (June 12th 2017)
Permanent pricedrop for both PS4 Slim and PS4 Pro in October.
PS4 Slim $249 (October 2017)
PS4 Pro $349 (October 2017)