If we draw an almost totally arbitrary line in the sand and call it "500 pixels per inch," then smartphones now stand proudly on one side of it, while tablets still languish on the other. Japan Display is gently nudging the market forward, however, with the 4K 12-inch tablet panel we saw last year (which offered 365 ppi) and now with a 4K 10-inch prototype that delivers a much higher pixel density of 438 ppi. That's good news for Chuck Yeagers who reckon they can spot the difference, but Japan Display is promising something even more important: It claims its 4K (3,840 x 2,160) screens have just the same appetite for energy as the regular 2,560 x 1,600 panels found in many tablets today. That means 4K slates could arrive at no cost to battery life, relative to current technology, leaving us with just the pesky financial and computational overheads to deal with instead.