I have an LG OLED, so it might be different for other TV layouts, but at least for me, the 4K stream was completely separate from the regular program. Meaning, I couldn't select it from the guide, or I couldn't go to the game and click on the Quality setting to manually change it to 4K (similar to what you would do on Youtube, click the gear wheel, and change the quality).

For me, another box popped up at the top right of the screen. The box was almost invisible, and you would mostly miss it if you weren't looking for it, especially considering you are looking at the bottom left of the screen where all the other settings are. That box says Watch program in 4K (or something like that) and once you click it, it'll take you to another completely dedicated 4K stream.

So, I hope this helps, and sorry I don't have pictures to help visualize this.