I have a 4k monitor which has both a usb-c input and a HDMI input. However, the HDMI input does not support 4k, only the USB-C does.
I am using the Macbook Pro 2015 13-inch Retina which has 2 Mini Displayports that should be able to output 4K.
There are USB-C to Mini Displayport adapters out there but they seem to be intended to be used from USB-C output to Mini Displayport monitor and not the way I would need it.
More information about the monitor can be found here (I also got the Hub): https://cforcedesign.com/collections/frontpage/products/cf015c-4k-ultra-slim-portable-display