Pretty sure the apps support 4k, otherwise why would they have 4k-specific apps? Also the Youtube videos especially DEFINITELY look better than 1080p and like belthazor23.rh I too get the *2 thing showing... HDR, no. Hisense really should have added a dedicated "info" button on the remote to show exactly what your screen is displaying at that time. And I HATE how none of the Youtube apps (not on XB1 either) just straight up let you pick the resolutions available to you, it's only automatic and then you need to click on the "stats for nerds" thing which is sort of cryptic.
A question I had though, is what do people do when playing 4k content through the USB? Most files are so big that they're not supported by the FAT32 format which I read the USB drive needed to be formatted to (at least that's what it tells me when I try to move them over to the device).