That support article isn't the most helpful, lacking a critical picture @RokuDanny-R of the special cable/Ethernet adapter for a 4k that has a USB and micro USB connector as well as network. You can find pictures with obvious internet searches.
Streambar is easy as it has a separate power supply.
There is a variant of Ethernet cable that can carry plenty of power but it is more specialist. Standard is called PoE or Power Over Ethernet, 48v, usually used for office VoIP phones, wireless bridges in offices etc. Not something that Roku would use (too expensive).
Not a Roku employee