For one, it tracks your heart rate much more closely. You can see your current rate through a complication on your watch face, and the heart rate app shows more details about your history. And you'll be warned if there are problems: Your Watch will notify you if it detects an elevated heart rate when you're inactive. Apple is even using heart rate data to analyze arrhythmias as part of an FDA-sanctioned heart study whose results will be available later in 2017.
Also, version 4 not only makes it quicker to start exercise, but tracks a lot more of it. In addition to supporting new activity types like high-intensity interval training, you can switch activities on the fly. If you regularly hit the gym, there's now support for syncing your Apple Watch with treadmills, bikes and other equipment. And expect to get more motivation -- there are daily you're-almost-there nudges and monthly challenges to keep you going.
The upgrade includes the obligatory round of new watch faces, including the psychedelic Kaleidoscope and a slew of Toy Story characters. However, the centerpiece of the bunch is a Siri face that displays timely info such as reminders, meetings and news. There's a Siri complication for other faces too.
Other big improvements are more functional than anything. You can browse your apps in a list instead of the signature honeycomb grid, and the multitasking menu is now vertical. There's also a reworked music app that not only has a more visual interface but also automatically syncs most of Apple Music's automatic playlists (the New Music Mix, Favorites Mix and Heavy Rotation) so that you have something to listen to if you leave your phone behind. Finally, if you owe someone for dinner (and live in the US), Apple Pay Cash can send money from your wrist.
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