USB 3.1 Gen 2 or whatever the fuck they're calling it now can do 4K with an adapter, but the thing is that not all USB to HDMI adapters are made alike. Some of them function as an external GPU, meaning that the graphics processing is done by the adapter instead of your computer, so their performance can be spotty. I've heard of USB 3.0 adapters that claim to output 4K over HDMI but I have my doubts about those. USB C to HDMI/DP adapters sometimes support native video output over USB C, but that exists outside of the USB spec and varies from manufacturer to manufacturer and model to model. Some USB C types use the USB 3.2 spec, and some use Thunderbolt which is a completely different and often incompatible spec. Some are also outdated and only use the 3.1 spec and won't work with a lot of functions of more recent USB.
Long story short, USB is SNAFU right now. If you want it to be easy like the USB of yore, come back in 6 years once we've finally figured out how USB C is supposed to work.