Yet the PS4 is selling through this year faster then it has in any other year, so shipped or sold through ps4 is doing fantastic. That you think PS4 shipments will drop by 50% next year is incredible. PS4 will ship around 14-15m units next year.
The 3.4m figure for Horizon is for the end of APRIL not June, Horizon should reach 6m no problem, games aren't so front loaded that they drop dead after 2 months.
I think the market for PS4 will reach saturation sooner than some are expecting, I could be wrong, time will tell.
"3.4 million copies had been sold by the month of June 2017 – of which, 915,000 were digital" - https://en.wikipedia.org/wiki/Horizon_Zero_Dawn#Sales
Sales prediction, PS4: 122 Million, Xbox one: 50 million, Switch: 105 million.