PS4K is cute, but as mentioned, it may put off those who don't have a 4K TV, and won't make much sense when the PS5K rolls around.
I think PS4.5 does a pretty good job of conveying, "Still a PS4, but better, yet still not quite a PS5."
I don't like PS4K for the same reason as you.
I don't like PS4.5 either though

It'll be PS4-'Some short meaningless suffix here'
It should be something neutral, a naming convention that can be carried over to the next cycle also. Like the iPhone 5 -> 5s, 6 -> 6s. Something reusable. ('K' doesn't work very well in that context either IMO).
Heck, Neo could work as an official reusable suffix. PS4 Neo. I don't know - something like that though.