PEAKVO2 vs N4x4: One Protocol or a Full Library?

By Cristian Serb · Updated August 17, 2026

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N4x4 and PEAKVO2 are both VO2 max apps, and they overlap on the Norwegian 4x4. N4x4 is a free app built around that single protocol, with wide heart rate sensor support and detailed zone coaching. PEAKVO2 runs the Norwegian 4x4 as one of eight protocols and adds built-in beep and Cooper field tests. Pick N4x4 if the 4x4 is the only workout you want; pick PEAKVO2 if you want the 4x4 plus a full library in one app.

N4x4 is a genuinely good, focused app, so this is not about which is better overall. It is about scope: whether you want one protocol done thoroughly or a whole VO2 max library. Both run on Apple Watch and both surface a VO2 max trend, so the honest question is how much variety you need.

PEAKVO2 vs N4x4 at a Glance

What N4x4 Is

N4x4 is a free app built around a single workout: the Norwegian 4x4, four 4-minute efforts with three easy recoveries, about 33 minutes with warm-up and cooldown. It does that one thing thoroughly. It reads live heart rate from Apple Watch and a wide range of external sensors, including Garmin, WHOOP, Polar, Wahoo, COROS, AirPods Pro, and Bluetooth chest straps, calculates target zones from your own max heart rate, and gives real-time coaching with spoken cues, haptics, and visual zone indicators (orange below target, red above). After the session it charts each interval against the target zone, and it adds streak tracking and a Lock Screen Live Activity. If the Norwegian 4x4 is your workout, N4x4 is a strong, free, focused choice, especially if you train with a specific heart rate strap.

What PEAKVO2 Is

PEAKVO2 is an interval training app for iPhone and Apple Watch, built around VO2 max, that runs the Norwegian 4x4 as one of eight validated protocols. Alongside it you get Tabata, HIIT 30/30, Billat 30/30, Ronnestad 30/15, VO2 Max 3x3, VO2 Max 5x3, and Tempo Run, each with full-screen color-coded phases, automatic transitions, a haptic tap at each switch, and heart rate zones. It runs natively on Apple Watch with no phone needed, tracks your VO2 max trend over time, and includes built-in beep and Cooper field tests so you can measure progress, not just train. It is a training app, not a testing service.

The Core Difference: Depth on One vs Breadth Across Many

N4x4 and PEAKVO2 make opposite bets. N4x4 goes deep on a single protocol: because it only has to run the 4x4, it can put everything into that one session, which is why its per-interval zone feedback and sensor support are so thorough. PEAKVO2 goes broad: the 4x4 is there, but so are seven other protocols and field tests, so you can rotate stimulus, mix short and long intervals, and measure yourself, all in one app.

Neither bet is wrong. If you will genuinely only ever run the Norwegian 4x4, a dedicated free app that does exactly that is a clean fit, and N4x4's wide heart rate sensor list is a real advantage if you use a chest strap or a non-Apple device. If you want the 4x4 as part of a varied VO2 max program, with field tests to track progress, the broader library is the reason to choose PEAKVO2.

Who Each One Is For

• Choose N4x4 if the Norwegian 4x4 is the one workout you want, you want it free, and you value wide heart rate sensor support and detailed per-interval zone coaching for that single session.
• Choose PEAKVO2 if you want the Norwegian 4x4 plus a full library of VO2 max protocols and built-in beep and Cooper tests in one app, on iPhone and Apple Watch, with your VO2 max tracked over time.
• Either way, both run on the wrist and both surface a VO2 max trend, so the deciding factor is simply how much variety and testing you want around the 4x4.

Keep Reading

• Norwegian 4x4 Protocol: Proven HIIT to Raise VO2 Max
• The Apple Watch Interval Training App Built Around VO2 Max
• Best VO2 Max Training Apps, Compared
• PEAKVO2 vs Intervals Pro: Which Interval App Should You Get?