What Do 4K and HDR Actually Mean?

Thanks to the latest generation of shiny new consoles from Sony and Microsoft, there's two fancy new terms to try and get your head around – 4K and HDR (or high-dynamic-range). And, confusingly, the different consoles all do differing amounts of each, from the Pro to a normal PS4, Xbox One to the Xbox One S and the undeniably more powerful Scorpio.