The rumors says that there will be games on the PS4K that runs better than the PS4... but still both systems are getting the same games... right?
Yes.
For example GOW4.... the developers will take advantage of the new PS4K, but they will also make it run on the original PS4 with some sacrifices... right?
Yes, but I honestly see the sacrifices to only affect the fps of the games, not their actual look (ok, maybe not with Ubisoft's games...) How can devs claim that their engine are optimized for consoles? (for example: The Division engine performed better than Unity/Syndicate's engine and had partially customizable settings on consoles too.)
SO... does that mean when GOW4 release.. it will have two versions one for the PS4K and another one for the original PS4 physically and digitally? because you can't adjust the setting from the console itself like PC.. it's different on consoles than PC the game itself is adjusted already for the console hardware.
Is that possible?
No. It won't have two different versions. It will be the same disc/digital game. It will just run differently depending of your hardware. Like how it runs differently on PC if you keep the default settings of the game. Depending of your system's specs, you will usually get different levels of performances.