What frame rate are you after? Because that's the only issue really if you're expecting more fps at 1440p or lower resolution than what the cpu is capable of giving. But even then, 9900k is capable of 100fps+, at least, in most titles. Only way of knowing what fps a cpu can provide is choose a somewhat cpu intensive game (not esports, easy to run games) and lower graphic details to minimum and see what you get and measure total cpu useage. You can sort of gauge the amount of frames to expect but at much higher details the 4090 is capable of doing.
So key here is resolution/details vs cpu. Will you be happy with say 4k maxed details with 160fps as apposed to 1080p/1440p 200fps and potential stutters from a tapped out cpu? Not exact numbers but you get the idea. Cpu provides the frame rate, graphics card does the visuals. 4090 should do reasonably well with 9900k if resolution and settings are not too low, cpu will suffer then.
See this regarding frame prerendering cpu is responsible for.