HIIT 4x4 is an Apple Watch exercise monitoring app for High Intensity Interval Training (HIIT). The app sequences exercise between high and low intensity intervals. The intensity of the exercise is measured as a percentage of a person's Maximum Heart Rate (MHR).

The HIIT 4x4 intervals are as follows:

- 10 minutes warmup at 60% MHR
- 4 minutes high intensity at 90% MHR
- 3 minutes low intensity at 70% MHR
- repeat high-low intervals 4 times (4 high intensity intervals of 4 minutes each = 4x4).

HIIT 4x4 uses a speedometer type dial to display heart rate. When the arrow is in the green zone then your heart rate is within the interval target. If the heart rate later falls out of the target range, warning chimes are issued until back in range.

Each person's MHR is different, and changes with age. A commonly used formula for MHR is 220 - age in years. A more accurate number can be discovered by using a heart monitor during all out exercise and observing the highest heart rate achieved.

HIIT 4x4 stores your workout history using Apple Health.

HIIT 4x4 is based on the High Intensity Interval Training 4x4 program developed by the Norwegian University of Science and Technology's Cardiac Exercise Group.