displacement is the shortest distance between initial point (starting point) and final point.
In this case, boy goes to north 2 mtr then west 4 mtr and then south 2 mtr. That means boy first go north 2 mtr then take left 4 mtr and then get back 2 mtr. That is finally boy is 4 mtr towards left from his starting point. Which is the shortest distance. Hence, displacement=4 mt