If they are both hooked up at the same time, it should likely appear as a second monitor in display settings, for an extended desktop across both screens, and you should be able to verify and configure the resolution of each screen individually from there. Some games can potentially encounter issues when using multiple screens at once though, so you might want to unplug/disable the one if you won't be keeping them hooked up like this all the time.
And back to what I said before, gaming at 4K resolution requires a lot of graphics card performance. A GTX 1060 is an excellent card for 1080p, and fairly good for 1440p, but 4K will likely bring it to its knees in games like The Witcher 3, unless you turn all the settings down on low, in...