Apple Watch Series 4

40mm 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.

LTPO OLED Retina display backplane

Low-Temperature Polycrystalline Oxide technology significantly improves display energy efficiency, permitting larger screen dimensions while preserving 18-hour battery longevity.

Electrical 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.

Consumer 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).

Fall 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.

Digital Crown with haptic feedback

Completely re-engineered crown mechanism containing 21% more parts that incorporates precise tactile clicking sensations generated via the Taptic Engine.

Apple S4 64-bit dual-core SiP

First custom 64-bit dual-core processor in an Apple Watch, delivering up to 2x faster application performance.

Apple W3 wireless chip

(minor): Upgrades Bluetooth from 4.2 to Bluetooth 5.0 for improved wireless throughput and range.

Universal 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.

Acoustic 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.

Infograph and Infograph Modular watch faces

(minor): Ultra-dense watch faces supporting up to eight complications, designed specifically for the expansive curved display corners.

Gold stainless steel casing finish

(minor): High-polish gold PVD stainless steel finish joins space black and silver.

Optical 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).

Standardized internal storage

16GB internal storage standardized across both GPS and GPS + Cellular configurations.

Enclosure thickness

Reduced from 11.4mm to 10.7mm (-0.7mm), making Series 4 noticeably sleeker on the wrist.

38mm and 42mm original case standards

Retired from frontline production in favor of 40mm and 44mm form factors.

Ceramic Edition tier

(minor): White and gray ceramic Edition models omitted during the Series 4 generation (returning later in Series 5).

Backwards band compatibility

Retained complete compatibility with older bands (40mm accepts 38mm bands; 44mm accepts 42mm bands).

50-meter water resistance (ISO 22810:2010)

Maintained swimproof rating with acoustic water ejection.

18-hour all-day battery life standard

Maintained 18-hour daily target despite 30%+ larger screen and more powerful 64-bit architecture.