What is the reason behind the PS4K?

[GoOnKid]

If the rumours are true Sony develops a better version of the highly succesful PS4. But why? The system already sells great, where is the need of a newer model? Why not ride on the strong sales and harvest all those fat, greasy profits? Please don't say that console manufacturers start building new consoles after the last one launched. As far as it seems, this is no PS5. And 4K resolution alone? If that's the reason behind it, do you really think you would be willing to pay a whole new console? Is this maybe somehow linked to PSVR? And if so, how?

Or does Sony simply need to do something regarding the launch of NX? Is the PS4K simply a strategic response to Nintendos next console?