Multiplying by 2 is the same a doubling. Since 4 is 2 * 2, to multiply a number by 4, double the number twice.
Example: What is 4 * 100? Double 100 to get 200. Double 200 to get 400. Therefore, 4 * 100 = 400. By doubling twice, we do the same as multiplying by 4.
