Witcher 3 rendering at a higher resolution

Solution
Nvidia's DSR or AMD's VSR. They allow you to set resolution multipliers in the driver so that you can pick a higher resolution in games (e.g. 4x multi @ 1080p = 4k). This is called downsampling.
@Salman Soomro, I fully agree that doubletake's answer is the perfect choice. I'd like to add that even though the highest quality (not resolution) menu setting is "ultra" there is an even higher level of detail; "uber". You have to enable it from the INI file. If you google it, there is a free utility out there that will enable it for you. Enjoy.