4K (or Ultra HD, as it's also known) offers a much sharper image than HD, but the only way you can experience it is with pay services like Netflix, Amazon Prime Video, BT Sport and Sky, select YouTube videos, or by spending a fortune on Ultra HD Blu-ray discs and a player to watch them with.
But that's about to change. In fact, we could see 4K TV broadcasts start as early as next year.
That's because new specifications have been added to the Ultra HD standard, which should mean more broadcasters can provide "enhanced video and audio in their products and services".
These specifications include support for High Dynamic Range (HDR), Higher Frame Rates (HFR) and Next Generation Audio (NGA).
HDR is a technique popularised by cameras. It selects the best elements from each picture and combines them to make one nigh-on perfect image. For the purposes of video, it adds a lot more depth to the picture, making it seem more lifelike.
HFR refers to frame rates of more than 50/60 frames per second, while NGA creates more immersive sound using "object- or scene-based coding" like Dolby Atmos and DTS:X.
The Digital Video Broadcasting steering board – the committee tasked with making these kinds of decisions – will make these specs available from next year. So technically, we could see an explosion in the number of channels offering 4K content. However, that's unlikely, at first at least, because the improvements will need to be adopted by hardware manufacturers as well.
Still, it brings us one step closer to seeing 'Planet Earth 2' being broadcast in 4K. And that's got to be a good thing.