Day 4 simplifies the formula down to two exercises: squats and calf raises, alternating every 10 seconds for the full minute - three rounds of each. Then the standard deal: a minute of rest, and back in for 3, 5 or 7 total sets by level. It is the simplest card in the program so far, and simplicity is not the same thing as mercy.
The alternation is the engineering. Squats are a big, expensive movement - full hips, full thighs, lots of oxygen - and ten seconds of them at speed puts a real dent in your legs. The calf raises that follow are lighter work, but they are not rest: your legs stay under load, just lower down, while your breathing tries to recover from the squats that never quite stop coming. Big effort, small effort, big effort - it is interval training folded inside the interval, and by the third squat burst the "easy" calf raises will have opinions of their own.
Squat to proper depth every rep - thighs at least approaching parallel, heels planted, chest up - and drive up fast; ten seconds rewards speed but only counts full squats. On calf raises, rise all the way onto the balls of your feet and come down with control rather than dropping. If the pace makes your squats shallow, fix the depth first and let the rep count be whatever it becomes.
Two exercises taking turns for one minute - your legs will not need a more complicated argument today.
Make it Harder: Touch your fingertips to the floor at the bottom of every squat - proof of depth, ten seconds at a time.
Make it Easier: Squat to a comfortable height above parallel and rise on both feet with a hand on a wall for the calf raises.
