Question Can asRock z370 pro 4 handle i9-9900k?

In a word, maybe.

In more words, the 9900K at launch required a Z390 motherboard, not for chipset features, but for beefed up VRMs (power delivery). Now, your motherboard's listed CPU compatibility says it will, but it becomes a question of that power delivery. I'm not sure as to the specifics on your board, but you could end up power throttling the 9900K. I would say that it is compatible, but I'm not sure it is advisable. That said, I don't have all the information so you might need to do a little more research. Or just go for it knowing that you might not get all the performance that the CPU has the potential for... but you'll get 8 cores/16 threads of performance so it should still be worth it. It just might not turbo all the way.

EDIT: Oh yeah, you're going to need to do a BIOS update, assuming you are still running the BIOS the board shipped with.