Prediction: Sony will have a portable that will play PS4/5 games

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Pyro as Bill said:
I don't see how Sony can unify their software given the power gap and I don't see how they can adequately support 2 systems.

If Sony tries to hold back on power to make a home and handheld closer in power then XB and gaming PCs are ready to step in.

How?

Sony will either be dropping a PS4 Premium or a PS5 in 2019 or 2020. The thing is going to at most have a ~12TFLOP GPU, and a CPU capable of running games at 4K/60FPS traditional displays, and 4K/120FPS in VR. 

That kind of power gap and focus will make it very easy to continue to support PS4 for quite a while (3 - 5 more years). There is nothing stopping PS4 from running a game built for PS5 at somewhere beteen 720p & 1080p/30FPS. The same would be true for whatever mobile product PlayStation delivers. If they go as far as to make something Ryzen and Navi based then it will most likely be even more powerful than the base PS4.

With the PlayStation building its API's around scalability, it is going to be very easy for developers in house, and 3rd party to get their games up and running across a wide range of PlayStation hardware. Sony developers will have a very easy time supporting the platforms, because they are only coding for PlayStation, and all PlayStations will be running on the same scaled hardware tech. This is not like PS3 + Vita + PS4 where they had to make and optimize a massively different build for each device. This is one build scaled to each device, with minor optimizations.