QuadHD displays are yesterday’s hot thing. The new point of attraction is 4K. At IFA 2015, Sony introduced its Z5 family—Z5, Z5 Compact, and Z5 Premium phones—with Z5 Premium sporting the world’s first 5.5-inch 4K (3,840×2,160) with 806ppi display, “next-generation” cameras, and some teensy-weensy fingerprint scanners. This is the first time ever that Sony has launched a trio of phones at the same time. They all share many features, but there is one thing that differentiates the Premium from the rest. The 4K display is a truly ridiculous world-beating panel, with vibrant colors and deep blacks. Unfortunately, the new screen tech was saved only for the Premium, while Z5 stays with the 5.2-inch 1080p unit and the Z5 Compact gets only a 4.6-inch 720p display. Aside from the 4K monster, the Z5 Premium flaunts a mirrored glass back in black, gold, and chrome. Z5 comes in white, black, gold, and subdued green with a frosted glass back, and Z5 Compact seems to be designed for the younger customer base, dressed in white, black, but also yellow and coral; these too with frosted glass at the back. If you’re picky about the size of the device you’re choosing for yourself, you’re in luck: Z5 Premium has 5.5-inch display and is 7.8mm thick, the Z5 has a 5.2-inch display and is 7.3mm thick, and the Compact has a 4.6-inch display and is 8.3mm thick. What’s your size? Specs-wise, the trio shares pretty much the same features. All three have inside Qualcomm’s Snapdragon 810 processor, up to 32GB of internal storage (expandable to 128GB by microSD), high-res audio chips, “up to two-day battery life,” and a fingerprint sensor built into the power button on the right edge of the phone, a position that feels more natural than...