Final answer:
Sachiko's belief that her mother would want a toy doll for Christmas is an example of egocentrism, a concept from Piaget's preoperational stage of cognitive development.
Step-by-step explanation:
The concept illustrated by four-year-old Sachiko believing that her mother would like a toy doll for Christmas is egocentrism. At Sachiko's age, children are in Jean Piaget's preoperational stage of cognitive development, which typically lasts from 2 to 7 years old. During this stage, children often display egocentrism, which means they struggle to see the perspective of others and assume that others share their thoughts, feelings, and desires.