A typical 4K monitor will display 1440p (2K) without noticeable scaling artifacts. It will display 1080p perfectly. This question concerned me when first switching to 4K for work and home computers.
There is no need to buy a low resolution monitor for better appearance or performance on resolution alone. You may choose a lesser resolution monitor for it’s other model specific merits.
MSFS has good display scaling and more options with nVidia GPU. So you can run at 4K scaled up on-game vs display scaling on monitor or GPU. The instruments are clearer to read with scaling vs lower res native. I care about instrument readability a lot.