It seems disproportionate to build a recommendation on a single published figure, so here is the reasoning.
A conventional home treadmill deck typically sits somewhere between 6 and 9 inches off the floor. Nobody publishes it, so that range is inference rather than measurement — which is precisely the problem. At the top of that range you are stepping up most of a stair riser, unsupported, twice a session, before you have warmed up.
Four inches is a curb. It is a step you already make without thinking, dozens of times a day. And because Sunny publishes it, you can check it against something in your house rather than finding out after delivery.
For context on the alternative: the flat walking pads on this site work out at about 4.3 and 5.2 inches, derived from their overall heights. So this machine has a lower step-up than a flat pad, and it has handrails, which is the combination that page exists to look for.