On Windows, you can, with a giant gotcha. Netflix need you to have all connect screen 4k to play ultra hd on Windows. I don't know if it is different if the screen is buildin.
Interestingly, if any montior you connect is not HDCP 2.2 you don't get 4K.
GeForce Stuff like Nvidia instant replay gets disabled if you have Netflix up in a tab - so close those before you game.