You have to keep in mind that engines and games get more advanced and more taxing on hardware. So, let's say a Zen2 CPU may be enough to run current PS4 games at 60fps possibly at 4K or dynamic 4K, but it may not be enough for the games coming out in 2019/2020 and later. Therefore, there is room for superior hardware from launch. One that maybe does 4K30 and 1080p60, and one that does 4K~60 and 1440p60. Heck and with 8K coming, that's your dynamic capable PS5 Super Pro (5K?).
It is only in the last few years that decent FullHD TV's have become mainstream, a good 10 years since being introduced. I fully expect decent 4K TV's not being fully mainsream for at least another 3-4 years. Lets not forget that a high proportion of gamers still do so in their own rooms and not in a communal living area, that along with the fact that 4K TV's, to be beneficial need to be higher sized compared to normal 1080p TV's....so I fully expect it to take some time before 4K becomes a norm in every day houses. Also don't forget that Bluray has been out for around 12 years now and consumers are still buying more DVD's than Blurays, so it makes you think...when exactly with 4K Bluray become more standard....
8K IMHO is a pipedream at this moment in time. Even if it becomes a thing, it wont be used in consoles for any time soon (ie. 10 years).
I'm sure they can source a CPU/GPU combination that easily allows for 60fps at 4K.
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)