Day 4 is Day 1 again, with a little more of everything and one new position. Back to the 3 / 5 / 7 set choice and 2 minutes rest, and the chair comes back out: 6 sit to stands, 12 side leg raises, a 10-count balance hold, 6 calf raises and 6 hip rotations. Every rep count on the card went up by one or two - the smallest step the program can take, and it takes it on purpose.
The balance hold is the newcomer, and it sits between the leg raises and the calf raises so the legs get a change of job halfway through the set. Stand on one leg, bend the other knee so the foot lifts behind you, stretch one arm straight out to the side and rest the other hand on the chair back. Hold 5 counts, switch legs for the other 5. That is a balance exercise, not a strength exercise: the standing ankle, the hip and the core all make small corrections the whole time, and those corrections are what balance training is. The sit to stands, side leg raises, calf raises and hip rotations do exactly what they did on Day 1, only slightly more of it.
Everything from Day 1 keeps its standard - heels driving on the sit to stand, toes forward on the leg raise, full height on the calf raise, big slow circles on the rotations. In the balance hold, use the chair hand as lightly as you can - fingertips rather than a grip - and pick a spot on the wall in front of you to look at; a steady gaze does more for balance than any amount of muscle. If you want the harder version, hover the fingertips just above the chair without touching.
An extra rep or two on every line and a new way to stand still - the second legs day grows by inches, and inches are the plan.
Make it Harder: Hover the chair hand above the chair back for the balance hold - touch down only if you have to, then lift off again.
Make it Easier: Keep both hands on the chair back for the balance hold and let the lifted foot rest lightly on the floor behind you between counts.
