The Apple Watch Series 4 was a smartwatch introduced by Apple in September 2018. It represented a significant update over its predecessor, the Series 3, with several notable features and improvements. Here are some key features of the Apple Watch Series 4:

  1. Design and Display: The Series 4 featured a larger and more immersive display compared to the Series 3. It came in two sizes: 40mm and 44mm, replacing the previous 38mm and 42mm sizes.
  2. Health and Fitness: The Series 4 introduced several health and fitness features, including an improved heart rate sensor with the ability to detect irregular heart rhythms (ECG or electrocardiogram), fall detection, and enhanced activity tracking.
  3. Digital Crown with Haptic Feedback: The Digital Crown (the button on the side of the watch) received haptic feedback, making it feel more responsive and allowing for a more precise scrolling experience.
  4. Performance: It was powered by the S4 dual-core processor, which provided faster and more responsive performance compared to the Series 3.
  5. Watch Faces: Apple introduced new watch faces that took advantage of the larger display, allowing for more complications and customization options.
  6. LTE Connectivity: The Series 4 offered cellular connectivity for making calls and using data directly from the watch, even when your iPhone wasn’t nearby.
  7. Battery Life: Despite the larger display and additional features, the Series 4 maintained a similar battery life to its predecessor, offering about 18 hours of use on a single charge.
  8. watchOS 5: It ran on watchOS 5, Apple’s operating system for the Apple Watch, which introduced improvements in notifications, health tracking, and more.
  9. Water Resistance: Like its predecessors, the Series 4 was water-resistant, suitable for swimming and tracking water-based workouts.
  10. Compatibility: The Apple Watch Series 4 was compatible with iPhones running iOS 12 or later.

The Series 4 represented a significant step forward in terms of health monitoring and design aesthetics for the Apple Watch. However, since my knowledge is up to September 2021, there have been subsequent generations of the Apple Watch with even more features and improvements. It’s always a good idea to check the latest Apple Watch models and features for the most up-to-date information.