I’d question that is directly related to MSFS. If he has a G-Sync capable display it might have had to do with this as he didn’t use the native resolution.
Maybe the displays framerate sync didn’t work well while being forced to a lower res but I’m not an expert in this case 
Other than that just try what 4k will do on your system - activate DSR in your Nvidia Control Panel if you have a NV card, activate DSR factor 1.78 DL and 2.25 DL - this will provide two higher resolutions globally on your system. Select them and test the performance
The performance in general is not 1:1 in this case as it’s also some ai based upscaling, but it will provide you some feel about that.