My monitor is 1920x1080 so I guess I would just pick a resolution higher then that in game and see which one looks best?
Yeah, you can do that, just be aware that as you increase the resolution you will need more and more horsepower in order to maintain a good framrerate. You're unlikely to be able to downsample from 4x 1080p aka 3820x2160 on modern titles unless you're running an extreme high end SLI setup. 1.7x 1080p which is 2560x1440 is a better compromise that won't require an insane amount of GPU horsepower. Far Cry 4 is a quite demanding game, so you may have to not downsample at all if you want to maintain ultra settings and 60FPS depending on which card you have.