i was wondering what the significance of question 4 in the Tanner Stage Calculator was when it asks where on my body is half my height.
Question #4 was added because many guys had problems with judging their proportions. From art, the "ideal" proportions of a man is where half his height is at his penis. This doesn't work with everyone. Some guys are long legged and others are long in the torso. For example, the Olympic swimmer Michael Phelps is longer in his torso than in his legs and that difference in proportion is believed to give him an advantage in swimming. But typically, the half-way point is near your naval during childhood. It moves down in stage 2. And in stage 3 it ends up below your penis because the legs grow before the torso. As you reach stage 4, it moves up again and usually ends up near your penis.
The calculator uses the half-way point if other indications are that you haven't reached stage four yet. After that it is ignored. Having the question gives another way of verifying the proportions that doesn't depend on guessing.