It's documented at https://en.wikipedia.org/wiki/4K_resolution#Resolutions so only only TVs with 3840 × 2160 pixels would result in 1.77∶1 (16∶9) display.
This isn't hidden anywhere. Just have to check the specs of any set you buy today.
The clamor over the change from 4:3 hasn't quite died down yet either.
So, I've recently confirmed that LG's entire line of OLED 4K TVs can't properly display 16:9 aspect ratio images without stretching them horizontally across the TV's entire 2160x4096 ~17:9 display. I even got them to send a technician to my house, and he verified that the TVs are designed to be that way hoping that people wouldn't notice. Personally, I'm pretty steamed that LG would sell such a display, in full knowledge of this defect, and madder still that they refuse to fix it.
If you haven't noticed it yet, try mirroring a laptop or computer to the TV so you can compare the screens 1:1. It became clear as crystal once I did that.