4x4 is a HIIT workout built entirely from the jack family: 30 seconds each of half jacks, jumping jacks, half jacks, side jacks, half jacks, side jacks, half jacks and squat jacks - eight timed intervals of continuous jumping, followed by a full 4 minutes of rest before the next set. That long rest is not generosity. It is the engine of the whole design.
This is interval training in its classic 4x4 shape: roughly four minutes of sustained work matched against four minutes of genuine recovery. The work block cycles through jack variations so no single movement pattern fails before the block ends - half jacks are the workhorse you keep returning to, jumping jacks raise the ceiling, side jacks shift the load laterally into your hips and obliques, and squat jacks close the block by adding a squat's depth to every landing. Then the 4 minutes of rest let your heart rate genuinely descend, so the next set starts from near-full capacity and you can hit the same intensity again rather than limping through progressively weaker rounds. High effort, real recovery, repeat - that is what makes it HIIT rather than just four minutes of jumping.
Stay on the balls of your feet with soft knees for every variation - eight intervals of landings reward light feet and punish heavy ones. In half jacks, keep the arm swing crisp to shoulder height. In side jacks, push off laterally and land with your weight centered. Squat jacks land wide and low; find real depth on each rep even as the block closes. Use the full 4 minutes of rest and don't cut it short - recovering properly is following the program, not cheating it.
Four minutes on, four minutes off. The interval doesn't get easier - you get faster at coming back for it.
Make it Harder: Add a ninth 30-second interval of jumping jacks to the end of the block.
Make it Easier: Step the squat jacks wide instead of jumping them and keep everything else on the clock.
