Noob here for 4k hardware. 
What hardware do I need to run MSFS smoothly at 4k resolution on a LG C2 42”?

I am not focused on having all sliders on MAX. The one that I would like to have on high/max is clouds. 
 

But I saw this video on Youtube, and if this smoothness is real and doable, then yes thanks 🙂

 

If on a budget, I would get a 5800X3D and 4070 Ti to achieve pretty much the same, if not better at busier airports, as what you see in that youtube video at 4K Ultra settings. If you are wanting an optimal system, I'd go for a 7800X3D and 4080, run DX12 + frame generation at 4K ultra and you should rarely see less than 80 FPS. If going for the best or you are thinking of going high res VR, I'd up that 4080 to a 4090.

The rest of the components are similar between options, with 32GB mid-high speed DDR4/5 (as applicable), 1TB+ M.2 SSD, of which 500GB is the minimum just for MSFS and a few good addons, and a quality PSU suitable for your GPU (750W+ for 4070Ti, 850W+ for 4080, 1000W+ for 4090.