Apple Watch Ultra 4 Review: Inside the Biggest Internal Overhaul in Years

Apple’s latest flagship rugged smartwatch has arrived, bringing with it arguably the most substantial internal update the Apple Watch line has seen in several years, and certainly the most significant redesign for the Ultra series since its inception. The new Apple Watch Ultra 4 introduces a vastly improved optical heart rate sensor, innovative audio intelligence features, and a dynamic Readiness score system designed to help athletes and everyday users balance training loads with proper recovery. Alongside promises of a consumer-friendly Apple Health app redesign rolling out later this year, the Ultra 4 positions itself as a major step forward in wearable health and fitness tracking technology.

While testing both the Apple Watch Ultra 4 and the new Series 12 alongside family members, experts and reviewers have focused heavily on the revamped optical heart rate sensor. This hardware update is backed by a substantial Apple study involving 1,400 participants, the majority of whom simultaneously wore competing smartwatches on their opposite wrists. The core question surrounding the new sensor is whether Apple’s ambitious performance claims hold up under rigorous real-world scrutiny on roads, trails, and gym floors.

What’s New in Hardware and Sensors

The Apple Watch Ultra 4 introduces a wide array of internal and external enhancements compared to its predecessor, the Ultra 3. Powered by the new S11 chipset—which incorporates architecture from the A20 Pro chips found in the iPhone 18 Pro—the device features a secure audio processing exclave dedicated to on-device audio intelligence. Battery life for the Ultra 4 has been officially rated at over two days, translating to approximately 50 hours of standard use under normal conditions.

For endurance athletes, Apple has revamped its GPS battery modes, introducing an Extended mode that stretches battery life to 25 hours while disabling the always-on display and running form metrics. A new Max Extended mode pushes GPS workout battery life to an impressive 45 hours without reducing the 1-second GPS update rate or compromising recorded heart rate data, though it relies on a 5-minute rolling average on screen instead of instant updates and disables custom workouts and splits. The watch remains available in its core titanium finishes of black and silver, accompanied by a fresh collection of bands, including translucent Ocean bands designed specifically for the Ultra.

The heart rate sensing hardware has undergone a complete redesign. The new sensor features an increased electrode surface area for improved electrocardiogram (ECG) readings, new photodiodes positioned below for enhanced workout data capture, and a new LED array dedicated to 24/7 heart rate monitoring. Despite these additions, the physical sensor bump on the back of the watch is actually thinner than in previous iterations. Furthermore, Apple has increased background heart rate measurements to every five seconds—providing sixty times more data points than before—and boosted heart rate variability (HRV) readings to every five minutes, marking a twenty-four-fold increase in frequency.

Readiness Score and Vitals Integration

The introduction of a dedicated Readiness score brings Apple into direct competition with established recovery metrics from Garmin, Google, Fitbit, Whoop, and Oura. Apple’s score ranges from zero to ten, where zero indicates a strong need for recovery and ten represents peak readiness to take on heavy training loads. Each morning, users receive a numerical value accompanied by one of four descriptive categories: Recover for scores of zero to one, Pace Yourself for two to four, Ready for five to seven, and Go For It for eight to ten.

These scores are driven by four primary pillars: fitness and workout activity, overnight vitals including resting heart rate, respiration rate, wrist temperature, and overnight HRV, daytime vitals such as daytime resting heart rate and HRV, and sleep metrics. Generating a initial Readiness score requires seven nights of baseline data with at least four hours of sleep per night, after which the watch trends the data against a 49-day historical window.

Unlike static readiness metrics offered by some competitors, Apple’s score is dynamically updated throughout the day. Completing a rigorous workout will cause the readiness score to drop immediately to reflect accumulated fatigue, while rest or a nap can dynamically adjust the score upward. Vitals tracking also monitors personal baselines, flagging potential outliers in pink and pushing notifications to the user if multiple metrics deviate from established norms.

Sports Tracking and Daily Performance

As a sports watch, the Apple Watch Ultra series continues to serve endurance athletes effectively across diverse environments, ranging from long mountain treks to everyday gym sessions. The device utilizes the standard Workout app, accessible via the customizable Action Button found exclusively on the Ultra lineup. Users can configure customized data screens, known as Workout views, and execute structured training sessions created natively or pushed from third-party platforms like TrainingPeaks and Runna.

While the built-in Apple Fitness app provides essential post-workout summaries, its native analytical depth remains somewhat limited compared to dedicated endurance sports ecosystems. To bridge this gap, many athletes rely on third-party applications like HealthFit to analyze metrics in greater detail or sync data across external training platforms. Mapping and navigation capabilities largely mirror previous generations, relying on native routing or third-party apps for advanced offline topographical mapping outside urban environments.

GPS battery performance during extended outdoor activities closely tracks Apple’s specifications. Testing over multi-hour cycling and outdoor sessions yields approximately 18 to 19 hours of continuous GPS tracking in standard mode under bright ambient lighting conditions, aligning closely with the stated 20-hour benchmark.

Audio Intelligence Features

The introduction of an Audio Intelligence feature set brings advanced on-device processing capabilities to the wrist. Functioning independently without requiring a nearby iPhone for immediate execution, the system currently supports features such as Sound Recognition alerts and instant song identification via Shazam.

Sound Recognition automatically notifies users on their wrist when specific ambient noises occur, such as sirens, doorbells, or a baby crying, making the watch a valuable tool for accessibility. Meanwhile, the upgraded Shazam integration operates continuously in the background using minimal audio signatures, allowing users to identify playing music instantly by simply raising their wrist. Additional capabilities, such as Live Rewind for text-only transcriptions of spoken words and Siri Recap summaries, are slated for release later in the year.

Heart Rate and GPS Accuracy

Apple has published a comprehensive white paper claiming that the Series 12 and Ultra 4 feature the most accurate heart rate sensing available in any wearable device, supported by comparative testing against reference chest straps. Independent analysis reveals that while the optical heart rate sensor delivers exceptional overall accuracy across steady trail runs, indoor cycling, and interval sessions, its underlying data recording methodology differs significantly from competitors.

During workouts, the watch records data points dynamically based on algorithm confidence, occasionally leaving short temporal gaps when confidence drops rather than plotting uncertain readings. While this approach prevents erroneous spikes from appearing in final data sets, it means that direct comparisons with devices that record fixed one-second intervals require careful interpretation. GPS accuracy remains exceptionally strong across both running and cycling disciplines, maintaining tight alignment with multi-band reference devices in challenging overhead environments following the removal of GLONASS satellite support.

Conclusion

The Apple Watch Ultra 4 represents a mature and comprehensive evolution of Apple’s premier rugged wearable. By combining a significantly refined optical heart rate sensor, a dynamic and responsive Readiness score system, and expanded background health tracking, the device offers a compelling package for both daily smartwatch users and dedicated endurance athletes. While certain software ecosystems and analytical depths continue to rely on third-party integration, the hardware improvements solidify the Ultra 4 as a top-tier option in the modern wearable market.