HDMI 2.0 is the connector standard on the TV and source. Initially when 4K came about the standard HDMI connectors could only display 4k at 30FPS, but now most 4k TVs come with hdmi 2.0 as standard (allowing 60fps and above). As far as cables go, for full 4k you do need to make sure you're buying a cable labelled as "High speed" (which to be honest the majority of HDMi cables are now anyway) because they do have a higher transfer rate to ensure no juddering or freezing in the image. Just like if you're trying to play 4k through a USB 2 connection rather than USB3. High speed cabling does not need to be expensive, you can probably get one for next to nothing. Cable length can become a factor if you need to run the cable a long distance, but I would imagine most people have their console right by the TV anyway.
However, one thing to note, there are not currently ANY games on any of the current consoles which will actually be 4k, yes a 4k TV can upscale, but that's not going to produce true 4k, the xbox one s, at the moment, will only be showing video content in 4k when the video source is 4k, not games.