watchOS 4 is the fourth release of watchOS, shipped September 19, 2017, alongside the LTE-equipped Series 3 — whose standalone cellular calling, streaming, and messaging it enabled, the watch's first steps off the phone's leash. The Siri face surfaced proactive cards ordered by machine learning, Workout was rebuilt with quick-start and auto-sets for swims, and GymKit paired the watch with cardio equipment for two-way data sync.

LTE support; the Siri, Kaleidoscope, and Toy Story faces; redesigned Workout with High Intensity Interval Training; GymKit; resting/walking heart-rate insights and elevated-heart-rate alerts; Apple Music streaming (4.1); and person-to-person Apple Pay.

Cellular independence recast the watch from accessory toward standalone device — the runner-without-phone scenario became the ad — and the heart-rate intelligence introduced here began the health-alert franchise (AFib, ECG, fall detection) that came to define the product.