To solve this question, the user needs to know basic arithmetic, the concept of multiplication and subtraction.

Let the sister's age be x.

Given, the boy is 4 years old and his sister is three times as old as he is. Therefore:

x = 3(4) = 12

So, the sister's current age is 12 years.

When the boy is 12 years old, it means that 8 years have passed since he was 4 years old. At that time, his sister's age would be:

x + 8 = 12 + 8 = 20

Therefore, the correct answer is:

The Answer is: C. 20.