- Introduced40mm and 44mm casing architecture with curved borders
First physical case enlargement in Apple Watch history, expanding screen viewable area by over 30% while slimming overall case thickness from 11.4mm to 10.7mm; display corners are curved to match the contours of the casing.
- IntroducedLTPO OLED Retina display backplane
Low-Temperature Polycrystalline Oxide technology significantly improves display energy efficiency, permitting larger screen dimensions while preserving 18-hour battery longevity.
- IntroducedElectrical heart sensor hardware (Digital Crown & back crystal electrodes)
Physical electrodes integrated into the titanium Digital Crown and an ultrathin chromium silicon carbon nitride layer on the back sapphire crystal, forming a closed Lead I bi-potential circuit when a finger touches the crown.
- IntroducedConsumer ECG app (FDA De Novo Class II software enablement)
Software application utilizing the built-in electrical heart sensor to record a 30-second electrocardiogram and classify normal sinus rhythm vs. Atrial Fibrillation (AFib). (Hardware present at launch on 2018-09-21; US software clearance granted 2018-09-11 via De Novo DEN180044; app launched via watchOS 5.1.2 on 2018-12-06; international rollout occurred iteratively across 2019–2021 as local regulatory approvals were secured).
- IntroducedFall Detection capability
Next-generation accelerometer and gyroscope sampling at 8x faster rates up to 32 g-forces; analyzes wrist trajectory and impact dynamics to detect hard falls and automatically call emergency services if the user remains immobile for 60 seconds.
- IntroducedDigital Crown with haptic feedback
Completely re-engineered crown mechanism containing 21% more parts that incorporates precise tactile clicking sensations generated via the Taptic Engine.
- IntroducedApple S4 64-bit dual-core SiP
First custom 64-bit dual-core processor in an Apple Watch, delivering up to 2x faster application performance.
- IntroducedApple W3 wireless chip
(minor): Upgrades Bluetooth from 4.2 to Bluetooth 5.0 for improved wireless throughput and range.
- IntroducedUniversal ceramic and sapphire back crystal
Black ceramic and sapphire back crystal becomes standard across all models (both GPS and GPS + Cellular), enhancing optical and electrical conductivity and radio wave transmission.
- IntroducedAcoustic redesign (50% louder speaker & relocated mic)
Speaker volume increased by 50% for clearer phone calls, Walkie-Talkie, and Siri; microphone moved to the right side between the Digital Crown and Side button to eliminate acoustic feedback and echo.
- IntroducedInfograph and Infograph Modular watch faces
(minor): Ultra-dense watch faces supporting up to eight complications, designed specifically for the expansive curved display corners.
- IntroducedGold stainless steel casing finish
(minor): High-polish gold PVD stainless steel finish joins space black and silver.
- ChangedOptical heart sensor
Replaced four-diode cluster with a compact second-generation optical heart sensor; added low heart rate alerts (in addition to high heart rate) and irregular rhythm notifications (FDA De Novo clearance DEN180043 enabled in watchOS 5.1.2).
- ChangedStandardized internal storage
16GB internal storage standardized across both GPS and GPS + Cellular configurations.
- ChangedEnclosure thickness
Reduced from 11.4mm to 10.7mm (-0.7mm), making Series 4 noticeably sleeker on the wrist.
- Retired38mm and 42mm original case standards
Retired from frontline production in favor of 40mm and 44mm form factors.
- RetiredCeramic Edition tier
(minor): White and gray ceramic Edition models omitted during the Series 4 generation (returning later in Series 5).
- RetainedBackwards band compatibility
Retained complete compatibility with older bands (40mm accepts 38mm bands; 44mm accepts 42mm bands).
- Retained50-meter water resistance (ISO 22810:2010)
Maintained swimproof rating with acoustic water ejection.
- Retained18-hour all-day battery life standard
Maintained 18-hour daily target despite 30%+ larger screen and more powerful 64-bit architecture.