Four by Four (fa7b3d)

Four by Four: VO2 Max Training

Norwegian 4x4 interval training is the gold standard for improving VO2 max -- and Four by Four is the only app built specifically to coach you through every second of it, live from your Apple Watch.

The protocol is simple: four rounds of 4-minute high-intensity intervals at 85-95% of your max heart rate, separated by 3-minute active recovery periods. Researchers at the Norwegian University of Science and Technology proved this method delivers dramatic cardiovascular gains in just weeks. Four by Four turns that science into a guided workout you can do anywhere.

HOW IT WORKS

Strap on your Apple Watch and tap Start. Four by Four reads your heart rate in real time and tells you exactly what to do. Speed up. Slow down. You are in the zone. Audio coaching cues play through your headphones so you never have to look at your wrist.

The app walks you through a complete session: warm-up, four high-intensity intervals, three recovery periods, and a cool-down. Every phase is timed. Every transition is announced. All you have to do is run.

REAL-TIME HEART RATE COACHING

Your heart rate zone is front and center. A large, color-coded display shows whether you are below, inside, or above your target zone. Audio cues adapt in real time -- if you drift too low, the app pushes you. If you spike too high, it tells you to ease off.

Zones are calculated from your age and resting heart rate using the Karvonen formula, the same method used in the original Norwegian research.

APPLE WATCH INTEGRATION

Four by Four runs a native watchOS companion app. Heart rate data streams directly from the watch sensor to the phone over WatchConnectivity. The workout is recorded as an official HealthKit workout, so it counts toward your Activity rings and appears in Apple Health.

WHAT'S INCLUDED

• Guided Norwegian 4x4 interval workouts
• Real-time heart rate monitoring via Apple Watch
• Audio coaching cues during every phase
• Personalized heart rate zones based on your age
• Warm-up and cool-down phases included
• Workout summary with interval breakdown
• Phase-by-phase timeline of your heart rate
• Interval consistency tracking across sessions
• HealthKit integration for workout recording
• No account required -- just set your age and go

BUILT FOR RUNNERS

Whether you are training for a 5K or building your aerobic base, the Norwegian 4x4 protocol is one of the most time-efficient ways to improve your cardiovascular fitness. Three sessions a week, 35 minutes each. That is all it takes.

Download Four by Four and start your first workout today.