What is the reason behind the PS4K?

[Lafiel]

if PS4K is real and has a clear edge in processing power (not just 4K movie support) then the reason for it probably is that the 28nm APU can't be shrunk without disproportional effort

the APU is the most expensive part of the console and reducing it's prize usually comes down to lowering the die size as wafer costs are pretty much constant and the more chips you can produce from a wafer the lower is the cost per unit

shrinking from 28nm to finfet (14/16nm) afaik is much more complex than it was before (and even before it could cost tens of millions of dollars), but AMD does have architectures ready for mass market that can be produced in the finfet process in Zen and Polaris, yet just slapping those on a die with the same specs as in the PS4 can result in games produced for the old APU not working as intendend (architectural differences), so you want to go with a chip that outperforms the old one in pretty much every stat to brute force through possibly less efficient execution of the game code

I think Sony may see this as an opportunity to use a clearly more powerful chip that can be sold at higher margins but costs the same or even less than the current PS4 APU to produce (especially when yields improve and the cost of using the new finfet process comes down)